First, let's forget about PnP D&D rules, the game makes enough changes to the original rules where such a discussion would be fruitless, and rather focus on the balance within BG2EE. EE introduces a very significant change with the new Grand Mastery into the fighting dynamics that in the original BG2 engine was only possible with a mod. To most players this will look like an obvious bonus that fighters SHOULD get, and to most old players of both BG1 and BG2 it might look like a welcome come back. In BG1 the experience required to get Grand Mastery is unobtainable in most game runs so it is less of an issue. In the original BG2, however, they specifically changed this and I think it is worth a deeper look why they did it.
Starting BG2, GM (Grand Mastery) becomes available to some warriors. As a compensation, Rangers and Paladins can choose to get 3 points into dual wielding at the start of the game together with specialization. For ease of comparison let's assume a fighter will start the game with 3 points into a one handed weapon and 3 points into dual wield and ranger or paladin will start with 2 points in one handed, and 3 points in dual wield. In a 6 person party GM will be available for let's say about 4/5 of the game together with dual wield. Now let's crunch some numbers for the original BG2 GM, BG2EE GM, and BG2/BG2EE warrior specialization (specialization is the same for both games).
Let's assume we'll use a one handed weapon that does 1-8+3 damage for an average of 7.5 damage per hit. Also let's assume that the character will have 19 Strength for +7 bonus to damage.
Damage per hit: 7.5 (weapon) + 2 (specialization) + 7 (strength) = 16.5
Attacks per round: 1 (main hand) + 1 (off hand) + 1/2 (specialization) + 1 (13th lvl warrior) = 3.5
Damage per round: 16.5*3.5 = 57.75
Now to translate the bonus damage of GM (+2 more compared to specialization) into a number we can use, let's multiply this +2 by the duration of the game that they will get it, or by 4/5.
Original BG2 Grand Mastery:
Damage per hit: 7.5 (weapon) + 2 (specialization) + 2*(4/5) (GM) + 7 (strength) = 18.1
Attacks per round: 1 (main hand) + 1 (off hand) + 1/2 (GM) + 1 (13th lvl warrior) = 3.5
Damage per round: 18.1*3.5 = 63.35
GM also gives +1 thac0 bonus. They receive this bonus for 4/5 of the game, but by the time they get to ToB the practicality of this +1 bonus disappears, because the Thac0 gets low enough that every swing is an automatic hit unless it is a critical miss. To take this into consideration let's assume that this bonus matters for 2/5 of the game. On a d20 roll +1 means 5% better chance to hit, and we can also translate this into +5% damage bonus. Throughout the course of the game this 5% will translate into (2/5)*5% = 2% more damage on average. Finally to get the overall Damage per round with GM:
63.35*1.02 = 64.617 or about 12% more than the damage with specialization
BG2EE Grand Mastery:
Damage per hit: 7.5 (weapon) + 2 (specialization) + 3*(4/5) (GM) + 7 (strength) = 18.9
Attacks per round: 1 (main hand) + 1 (off hand) + 1 (GM) + 1 (13th lvl warrior) = 4
Damage per round: 18.9*4 = 75.6
GM in BG2EE gives +3 thac0 bonus, or +2 more than specialization. On a d20 dice that's 10% better chance or 10% more damage for 2/5 of the game, or 10%*(2/5) = 4% more damage on average throughout the whole game. Using the same calculation as above:
75.6*1.04 = 78.624 or about 36% more than the damage with specialization
To summarize, original BG2 GM gives 12% more damage than specialization vs 36% in BG2EE. This begs the question whether there is enough bonus abilities that a Paladin and Rangers receive in order to compete with the 36% superior damage of a Fighter. In my opinion there are a lot of ways to create an overpowered character, but this becomes a very big deal for the NPCs. I believe this pushes the balance in favor of the three NPC warriors in the game and discourages the use of certain classes too much.