Honestly if you're that concerned about head flow you should go to aluminum heads....because all other things built up, the iron heads ARE going to be the bottleneck...aluminum 3x00 fwd heads stock outflow the best ported iron heads possible, and since you have a 3.4 the swap is easier than a 2.8 or 3.1, because your engine is already setup to run DIS, just my 2 cents, imo keep the porting you have and use that money you would have spent on a high end shop to start getting things for a hybrid build..
its pretty much what I'm doing...I built my engine up with iron heads, but once the rest of the car is up to par I'm going to build it again as a hybrid...I need to rebuild the lower end still anyways, 140k on my block and rotating assembly, top end is rebuilt so pretty much new..and all that said, I have no idea if supersix is any good or not, haven't heard of them but its not like I make it a point to know about speedshops either, I do know that for aluminum head porting WOT tech while expensive probably has the best stuff to offer.

 

I will probably cave to the superior flow of the 3500 heads at some point, but since I already have my heads done...they have been sitting a bit, started this before the hybrid craze... I was hoping to have them looked over by someone I could get them to. Really I just want them flowed

 

Imo do what I did, just bolt them up knowing they're better than they were and don't worry about just how good they are, because you know you're going to get something even better.