I have thought about this in the past. In my opinion, it would be worth it. Yes, the parasitic drag would be significant, but isn't that the case right now? We are already losing 40-70HP depending on modifications and still seem to benefit from a single supercharger.
The problem I ran into is physically fitting two M90's within or around the stock fuel rail. The stock intake, rail, thermostat neck, etc... are all designed to nestle a single M90. When you try to shove two in there it gets way to tight. You can always go around that though by lifting the blowers up higher, but I can't imagine a longer SC belt doing much for efficiency. Not to mention, the only way I found it possible to put two M90's under the hood would require at least a 8 inch cowl........which is getting a little rediculous.
You couild probably install two M45's at a 45 degree angle to fit where the single M90 was, but you can do the math on that one. It would be pointless and probably more harmful than a single M90 due to the drag.
A M112 would fit....especially the 4th gen M112's cause they are even more compact. With an internal bypass it would just be a matter of blocking off your old bypass bung in the upper intake. You would have to measure the correct nose drive length, but it seems like it will fit alright. The custom work involved in this mod for just another 22 cid blower however its a little discouraging.
If you want to take on a custom blower project, the Whipplecharger 2300 series (can't recall the exact model) is a good fit and would probably work just as easily as the M112. It would for DAMN sure be worth the trouble. But be prepared to have an absolute torque monster. It would make ALL of its torque with the snap of a finger and be fairly spent by 5500rpm.
As for turbo projects..........the only problem is that our cars are not designed for high revs needed with a turbo as stated above. If you built the car accordingly then it might be ok, but I'm not so sure how streetable it would be. To get the kind of power needed to plant you back in a 4000lb car I imagine it would be kinda dead feeling down low and then ape **** up high. Just ask Ted Lyons about that kind of power......it's not very fun. Gets pretty aggrivating too when a street car with half the HP can give it a run cause it's got nothing down low and the going sideways up high.
I'm convinced that the aftermarket currently makes available everything I want out of my car. Numbers well over 300HP/400T at the wheels are being achieved now and the way the power is being made is my cup of tea.
Micah