Darts get no love in this game. The one NPC with a clear love of darts is used as semi-comic relief early in the first game, but yet he inspired me! What is the maximum potential of this neglected weapon?
Well, we'll start with the good news: 3 attacks per round. We will want to hit hard and often, so looking for +damage to boost our output, each boost is automatically tripled! And we can do this single-weapon style or while holding down a shield, for an early AC boost.
Then the bad news starts - no strength bonus. We do get a great THAC0 bonus from max dex early in the game, but dex is the least boostable stat over the length of BG saga - I have no idea if it plays any better in IWD, but I get the impression that has significantly fewer stat boosts, so STR-based weapons might not get ahead as easily?
However, we can get a weapon specialization bonus, for 1+THAC0/+2 damage for any impure fighter class (such as a paladin or a multi class) and a whopping +3/+5 for 5 pips as a 9th level pure-breed fighter. This comes win a bonus 4th APR, and as we are straight fighter, we pick up another 1/2 APR at 7th and 13th, leading to 5 APR (unhasted) doing +5 damage - but can we do better?
The obvious place to look is kits, but we are spectacularly unlucky. As a fighter, I would want something like the Kensai kit, which grants extra damage every few levels for melee weapons, but bans range weapons. It happens to boost throwing weapons that double as melee weapons, which is every other throwing weapon in the game that is not a dart, oh, and they get the strength damage bonus too. So no help there. Never mind, Archers provide a similar bonus for missile weapons, but guess what? Throwing weapons are not missile weapons either - so dart is the ONLY weapon in the game that can benefit from neither of those significant boosters. Still, all is not lost!
First, we can play an assassin. That gives us an extra +1 THAC0 and +1 damage from level 1, plus the use of poison which plays super well without many APR. So option (1) is a 2 level dip into assassin, dealing at the first opportunity so that we do not lose out on too many fighter hit points, and giving up any idea of a fighter kit (they don't help darts builds much anyway).
Option 2 is a longer-term bet that will not pay off until late into ToB - swashbuckler. Swashes get +1/+1 THAC0/damage bonus every 5 levels, so +5/+5 at 25th level, costing 3.3 million xp. 26th level fighter will run you another 4.5 million xp, totaling 7.8Mxp, just under the 8 million cap. It is possible! But highly unlikely solo, and gives up a lot of HLAs. Also, you are not going to get 27th level fighter, that is 50kxp over the xp cap. But I think that is our ultimate build for unassisted damage output - +8/+10 THAC0/damage at 5 APR from weapon style and dualed kit. You will see this so late that it will not be relevant, but may be fun to cross off on the "Hey, I did it!" list. Console jockeys can have fun too.
A more reasonable dual from swash at 20th means "only" 2.2 million xp into Swash, and hitting 21st fighter at a further 3,250,000 xp. I have had a 6 person party hitting around 5Mxp completing SoA with a good deep dive into Watcher's Keep, so this is totally doable for a straight BG play through, although unlikely to repay the dual until early ToB. The extra 5 levels of swash seem unconvincing for an extra +1/+1. Conversely, taking swash to 40 for +8/+8 also seems a poor choice, as you give up the weapon specialization. You do not even get the +1/+2 from basic specialization playing a Swash, as darts are still not melee weapons. Finally, you will have only 3 APR rather than the fighter's 5, although you are still getting more than half the attacks, so it is not a big a penalty as for builds around more useful weapons
Of course, the next consideration is: is there any good kit to boost the weapons for me? The most obvious is the gauntlets of weapon specialization, which give a further +1/+2 in BG, where they are easily obtained towards the end of act 3, or a +2 THAC0 bonus, if preferred, from the aforementioned comic relief. Unfortunately, the BG2 version of those gloves is buried deep in Watcher's Keep, and comes with a bonus 1/2 APR that you won't be able to use (if fighter) as the game caps APR to 5 without magical assistance (enhanced haste only?)
Speaking of improved haste, Cespenar can make an improved cloak of protection which casts improved haste once per day, pushing us briefly to 10 APR. Likewise, the ring of gaxx will give us another 3 castings per day.
In the helm slot, the Helm of Balduran can give another +1 THAC0, but no extra damage.
Not much notable in the armors, although Shurrupak plate packs a +1 dex bonus, which may or may not be helpful.
Nothing great jumps out of the amulet, belt, boots, or shield slots, although the shield of Balduran -1 STR penalty is not going to be a concern when chasing down beholders with darts
For cloaks, the other strong candidate is cloak of the stars, to make 6 +5 darts/day. Alternate cloaks to make the most of their abilities.
Finally, we come to the darts themselves. Stock a variety of special purpose darts that do +1d6 damage of various elements, or cause wounding etc. These are more useful in BG2 and later, as the high APR barrage of damage is enough for most non-magical foes in BG1.
For BG2, there is only one game in town - make a beeline for the first floor of Watcher's keep and the +3 returning crimson dart, which will be your mainstay for the rest of the game.
So at the end of the day, is a dart build feasible? Well, generally any build is feasible in this game, and it is certainly not awful. I have a lot of fun playing my darts builds. The most glaring omission in the BG series is the lack of any distinct boost at all focused on darts. It would be nice to have just one new kit added, perhaps for halflings, that gave the kensai/archer bonus for thrown weapons. Then it will still likely be a poor relation to thrown weapons with a STR bonus, but would be closer to the fore of the crazy builds.
Main thing in favor of this build is high APR multiplying any damage bonuses, and novelty. Down side is darts do piercing damage, very little damage without bonus, and even have a poor range compared to bows - you are not sniping safely out of harm's way.
Go forth and have fun!