Most efficient mod for $$$$$$

[ChrisZang]

In case you are interested in improving your cornering speeds and look for a "budget improvement" then here is my tip:
Porsche set up the Boxster to understeer (a.k.a. pushing), my guess is that they figured that the average driver can handle a car which mildly steers towards the exit of a corner better than a car where the back end comes loose (a.k.a. oversteer).
IMHO they overdid this a little.
So I would suggest:
a) get wider front tires, go from 205 to 225, they fit on the existing rims
b) go to an alignment shop and get as much negative camber in the front as the stock suspension allows you to do (probably 3/4 degree)

Both mods together will give you much more grip on the front and allow you to take fast turns much quicker and your Auto-X and DE/TT times will improve with this simple mod.
Yes, and in case you do not Auto-X then starting Auto-X will be an even cheaper performance improvement.

Cheers