Bruce, there is no need to boost a Spyder. I know I'm a little late but I've been busy lately.
I know Greg is building you a special chassis with IRS(and people this is NOT the same as a Bug IRS), but is he also getting rid of the front beam? The beam is your limiting factor.
The car is smaller and lighter than any Speedster(except that one with an aluminum frame). Others have spoken and I agree with them, namely Ed and Marty. A balanced car is better than one with too much power. Brakes, handling, and how the car behaves over bumps(especially variations in the road) are all as important as gobs of power.
Bob, Marty, and Tom all have the 911 suspension in their IMs along with that beautiful frame Henry makes. Tried and proven.
Your car will be a guess at best as to handling when you first get it. This is over 40,000 miles of seat-time in a Spyder talking, some of it at and over the limit. Part of that limit is tires, as you can't get as much stick as the 911 IM has. Trust me on this, the life you save may be your own.
Anything more than 200hp is a fool's errand in a Spyder. 170-180 is just right IMHO.