@Fatal, I only run 2s as 3s is a recipe for something to get broken.

Yes I think the stock shocks are too soft as well. LC Racing makes thicker 1.2mm and 1.3mm shocks for the front and back - plus thicker oil really helps the shocks.

As far as under steer... anything at full throttle will probably under steer, esp since this car is so light. If you put heavier diff fluid in the rear, it will make the under steer even worse. If anything I would think putting heavier fluid in the front would help drag the car around corners instead of pushing from the rear.

Since you have already messed with the wing mount, you can actually double up most parts on the car if you want to make it bash "proof". The front tower, rear tower, and deck brace are all ripe for doubling up.