The Exhaust and intake are the obvious mods that you can't do without.
Your car from what i gather is essentially going to be an alternate route for stage 3 and 3+ guys and its the smaller, more expensive modifications that are going to make all the difference i think.
You have the WMI which is good for a 10-15hp power gain (will you be mapping for it?), but will you be going down the APR cylinder head upgrades. Seems to have worked wonders on DanGBs and StevePs cars.
Steveps enclosed intake makes that ITG in h4ardy's post look tiny. If your going this route surely bigger is better?
Presume you will go decat again as well?
Whats your aims. guessing 450hp ish