You can make more power than 420 on a std head, the point is though you will reach a point where gains will be getting smaller and smaller for more more and more boost.
I always balance cost versus performance gain, no pint in a modded £1K head for extra 20bhp for example if a slight boost tweak will do it.
Always keep the cr as high as you can get away with, if injectors are near limit rail pressure can be increased, some cars(porsche etc) are 5 bar as std, with vac on, so 5.7/8 with vac off.
Proper mapping will be mind blowing driveability wise, and extra advance lower down will lose a lot of the lag, not all obviously.
tabetha