You have no option but to upgrade the clutch. The factory toyota clutch will not fit as it is designed for the dual mass flywheel.
The Toms and TRD light weight flywheels are designed for the TRD clutch kit - im unsure about the other models for flywheel.
So basically the whole operation cost about $2200, for the flywheel, disc and cover.
The HKS RS induction probably killed a little off the very low end (this is to be expected tho), the exhaust made a slight improvement through the rev range. The flywheel has offered the best improvement.
In saying that, there would be no way I would go and do it without first having a stuffed clutch.
With the Altezza it is recommended that you replace the flywheel when you do the clutch, this is due to the dual mass design. The factory dual mass flywheel is $1800 from Toyota, and on top of that you still need the clutch kit.
Toyota will charge you about $295+ to install the kit.
Cheers
David