Jim, go here:
http://www.toohighpsi.com/index.htm
Demon. you are going to need something to retart your timing under boost. also you are going to need a cage.
as for overkill? yeah i think so. why not a single turbo? check out the above link. if you insist on twin turbo, i would definetly get a stronger block or dont run so many PSI's.
Also Jim, when was the last time you seen a diesel rev up to 6 grand with a turbo? they are not for "high rpm" use. it is the design of the turbo is where(powerband) you can use it.
I do agree with Jim though, you have to decide what you are going to use this car for first, or you will be owning a money pit. read my link and check out the "bone yard" turbo set up. just gives you an idea of what you can do with a little money.
Turbo's are more efficient than S/C. the exception maybe with a roots type.