I still think that your car should track better than you describe.
With 350RWHP your car should be a handful, but with a 4spd and LSD it should still track straight. My car has ~300+RWHP but has a much lower geared 915. The torque multiplication of my 2nd gear is about the same as your 1st gear. I also do not have an LSD.
You track your car and should know fellow reacers with similar setups that can give you some input. The common problems that cause the symptoms you describe are low tire pressure, worn tires, worn suspension components, broken front swaybar, bad SLD, things like that. I assume you have checked those things.
Have someone watch your car from the rear and report what they see.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8