I think Kensai kit should get an AC buff, preferably ability to wear some light armor. Here are my thoughts:
1) Kensai is a fighter class, but has THE WORST AC of all classes including thiefs and mages. There are some better robes and braces for mages and some armor for thiefs, monks get AC progression. Kensai must be in the middle of battle (otherwise useless) and has to deal with the worst AC. Given that hits depend on stats and not player's ability (to dodge, for example) it can make even simple encounters (bandit archer circle) extremely frustrating even on higher levels.
2) Kensai gets +1 to hit and damage every 3 levels. It grants +10 bonus on level 30, but it is only gonna be +6 at level 18 by the end of SoA. Given that pure Kensai can reach maximum 8th level in BG1, the whole bonus is +2 to hit and damage FOR THE WHOLE GAME sacrificing ability to wear armor and braces (full plate 10AC, gauntlets of specialization +1 to hit, +2 damage). In BG1 maxed out kensai only gets +1 to hit over fighter (with his gear) while sacrificing 10AC and ability to use ranged weapons! Plus, a thing about berserkers immunities!
3) Kai ability is trash. Average damage for Katana is 5,5. Max damage is 10. Kai give maximum damage output for 10 seconds (one round is 6 seconds). Assuming you have 4 attacks per round, you make ~6 attacks in 10 seconds, so average damage difference is 27, assuming every strike hits. Is it worth sacrificing armor and bows?
4) Kensai looks silly the whole game without armor running around without pants
5) Those who do not like the idea can restrict themselves or not download the mod (in case someone decides to implement it).
I would suggest giving them ability to wear everything up to studded leather. It makes sense it is the least restrictive armor type and it is unlikely to make them overpowered at any stage of the game. Plus, it will make them more feasible for soloing and keeping pure class till the end.
TL:DR: In BG1 maxed out kensai only gets +1 to hit over fighter (with his gear) while sacrificing 10AC and ability to use ranged weapons!