The wastegate must be able to be open at "no boost", which mean the ubiquitous pneumatic 'gate is useless, except to prevent overboost.  You have to be able to open the throttle fully to get atmospheric performance, and then close the gate to bring in the turbo.  I can't see anything that will work for that except a manual gate, or perhaps something MUCH more sophisticated than we can build.  

This would be true if you insist on having the ability to run with no boost at all.  I'm not sure any wg will be big enough to pull that off though.  I'm perfectly content to set 40" MAP as my normal boost level, which will work fine with a pneumatic wg.  I'll still have to come up with a way to control this at altitude, but that's the least of my problems at the moment.   

You would like to have the turbine sized so that even with the gate closed at WOT, you cannot overboost.  This requires the turbine swaps [tuning] I mentioned earlier.  

This goes back to your definition of "overboost".  If I want 40" MAP at 18k ft, I'd be happy to have the wg closed fully to get that.  Of course that same closed wg at any lower altitude will easily exceed my 40" MAP limit.   

Most folks will be really happy with the performance 40-inches MAP will provide.  50 MAP is certainly safe with AV-gas and an intercooler.  

I suspect this is a very correct statement.  I had a long talk with Turbonetics Friday, and should be getting an official quote for a new turbo Monday.  The guy I spoke with REALLY knows his stuff when it comes to turbo aircraft, so I'm going to be far better off with the turbo that he's suggesting.  I'll have an external wg, which can be sized as I desire, so there should be no problem controlling the boost.  The manifold and turbine housing will flow so much better than the stock Mazda unit, that I expect 40" MAP will provide considerably more power than it currently does.   

Also, I agree that proper sizing is everything.  I talked about the RX-7 some with the Turbonetics guy, since they sell lots of turbos for those also.  He said that I could bolt this "aircraft" turbo on the RX-7, and I would hate it.  He said it would make big power at high rpms, but would generally be a dog on the street.