"After debating for weeks I finally decided today to install my short shift kit, WOW what a difference! Now the ECU tuning, wheels, headers and exhaust and i'll be SET!"
This is just based on my limited personal experience doing mods to my 2000 2.7L Boxster. Go for the ECU tune and wheels if you are after performance increases. I found a 15+ hp increase by going to ECU tune and intake. I would be careful with messing with the exhaust system unless your primary goal in a different sound.
So far my experience has been that the performance increase has been very modest when going to headers and high flow cats, maybe 4 hp on the dyno (went from 199 to 203 hp), and the exhaust mods are very expensive, have fun,
Ed