Beating a new car like a Boxter in autocross is difficult but not impossible.Even before more power I would replace <all> bushings on your car ( to include rack even if it still feels ok).Then new shocks, and your 3/4's of the way there.Maybe bigger sway bars too, tehy would raise limits but could make handling trickier.
A stronger brake set-up ( Steel braided lines vented rotors etc) and the stickiest tires money can buy.A well sorted SR, CA or even KA-T will give you plenty of power to compete with a Boxster.
As for the quarter you can beat a Boxster S all day long, just with a well sorted SR.
A well kept GT-s should be close enough to a new boxster in handling that it comes down to driver.Experience can be a Killer here.
Good Luck keep us posted please.
Fred...
