No point changing the intake manifold, waste of $$ and time. You will actually loose mid range power and torque.
Don't stress about the head gasket also. One thing you should invest in is CTR or DC2R springs and retainers or if you have the $$ buy Toda valve springs and Portflow retainers.
Also a tunable ECU like an Apexi Power FC for DC2R (you will need and adapter harness
to fit to your car, talk to Yonas at JDM Yard)
You will need the uprated valve springs and retainers so as not to get valve bounce in the high RPM region.
A decent header is also a good investment as are adjustable cam pulleys. These two mods will take advantage of the cams to the maximum.