What he said. Plus the extra expense. You can use that money MUCH better in areas like a better exhaust maifold. You want power, that's the area you need to be looking at. Thats the bottleneck. I think even below 800HP it counts.
Talk to SAAD or SP about a tubular manifold. Talk to Kirk at HPF about a tubular mainifold. It would be nice that the installation is all part of HPF build so your tune is pure HPF. It really helps if you have any warranty issues. Stay with HPF as much as possible, but tell them what you really want.
That 6766 makes mad power on SUPRAS which have better flowing exhaust manis an better flowing intake tubes.
If it all gets too complicated, then just let HPF do their standard build (no sleeved block) and DRIVE your car. The chances are good you will be pretty darn happy. You can always fiddle with manifolds and intakes way down the line.
But run the idea past HPF and see what they can do for you with being too costly and time consuming. You can easily shelve that idea for much later if now does not seem like the time for all that.
Good Luck. And dont stress. I think HPF can make that 6766 do everything that you need. They are good guys who want to please. And they are very capable.
One more thing. i ALWAYS like a baseline Dyno run with NO air cleaner no matter what the vehicle. I do this with Motorcycles, cars, FI, NA, etc.
It gives you very good picture if your air filter is restrictive. I think Marcus was loosing 50 PLUS HP with his airfilter. He has a bigger build so the figure is going to be pretty high. But you get the idea. I recommend the AFE air filter that has a hollow center for even more surface area. If needed, that is. Changing air filter models or brands is often the cheapest way to pick up some significant power. You will never know unless you do that Dyno pull with no air filter.
I could be all wrong about needing a tubular manifold and better intake breathing. I am going on logic not much else. I would very much like Larry from SP to chime in here and comment. I trust his judgment. I will ask him nicely to participate.
I feel I was talking a bit freely with no real hard facts behind me. Larry is the guy that this thread needs now IMO. I emailed him.
It's the weekend...a long one at that. So, I dont know exactly when we will be hearing from Larry. When we do, listen!