The hemi heads are better when you get into highly modified builds, but they require alot of port work to make them flow better than the old style predator head that you have now. If you have the head with 27/25mm valves then its likely an RTC-6 or RTC-8 head, which are my favorite heads when it comes to these engines. When you compare the head like you have now against the hemi head or even Honda heads, the RTC heads like yours flows a good bit more than the others in stock form, according to flowbench results that I have seen done by another builder. Add a little port work to the RTC head and you have a very nice head that will also accept aftermarket rocker assemblies that are already available, like the Champion rockers and several types of roller rockers out there now. The hemi head just recently had a set of aftermarket rockers available, which are the ones that NR Racing makes, and I believe there is one other company that makes them also but can't remember the name. What you have now is just as good as anything else to work with and build from, I would stick with it if it were me.