i dun think higher rev = higher hp... in general that is not true..
however, with the mivec head the camshafts profile is not the same as the standard gsr.. therefore you will have peak torque at a higher rpm range compared to stock gsr head.
just make sure your mechanic knows what he is doing if you want a good mivec+gsr hybrid.
timing belt is not the same. now u need an electronic device to activate mivec... fuelling & ignition map should be tuned as well. not so easy as it seems.... if you want the mivec exhaust sound.. i have to disappoint you.. in a turbo engine you can hardly hear the mivec engage.
all in all... i think the GSR tomei 270deg camshaft will do a better job if you are looking for top power.