Ok basic number crunch.
Lv 20 fighter, with 30Str, WF,WS,IWF,IWS, IC., PA
THF using a greatsword (+5)
TWF using a double sword (+5/+5)
THF:
(+37/+31+27+22), 2d6+19
TWF:
(+35/+30/+25/+20), d8+14
(+35/+30/+25), d8+9
Damage quoted as:
Without PA (With optimal PA)
Enemy ac 40:
THF: 65.325...