Current: Since BGEE is using BG1's proficiency tables, there isn't that much point to Fighters investing past Mastery, at the moment. The most substantial bonuses come at two and three pips, and all that is to be gained from the last two ranks is a total +2 damage and -3 speed factor.
Requested: As per PnP and BG1's manual, re-instate the extra 1/2 APR to Grand Mastery, to provide a nice bonus for investing five pips in the chosen weapon.
Games affected: BG1, BG2, BGEE.
I've attached a modified, tested copy of WSPATCK.2DA containing the changes. Feel free to use, if you're interested.