Wouldn't it be better to watch boost, inlet air temp, timing and mixture and
not track MAP?

I don't really understand what you're asking here.  MAP is boost when it gets above ambient, and certainly there is a need to monitor the other parameters.  Are you saying to quit worrying so much about limiting the boost to such a low number if everything else is in range?

How much boost did the stock Mazda run on the street? How much advance did
the street cars have dialed in at full tilt? 

I have no idea what they ran for timing on the 2nd gen engines.  The stock boost was 5.5 psi for the 87-88 S4 turbo, with 8.5 cr rotors.  The 89-91 S5 turbo ran 6.5 psi with 9.0 cr rotors.   10 psi is pretty common for both on the street with just a few bolt-on mods.  Now that I've looked at the rotor chart again ( http://www.mazdatrix.com/r-rtrwgt.htm ) I noticed that the 89-91 seems to use the same rotors as the 93+.  I know more about the 3rd gen engines, and know that folks are running these way up in the teens of psi for boost.  I'm starting to believe that I could just retard the timing a few degrees, and make sure I'm slightly on the rich side with fuel, and let-er rip to 6-7 psi.  Maybe I'm making too much of this 2-3 psi that Bruce suggested as a maximum.  Gotta think about this some more.

Perhaps a small manual waste gate to get into the range. Or a popoff valve in
the intake ahead of the injectors. Whatever it takes, you will get it right.

Thanks for all the info.  I really do want to keep the turbo, since this IS supposed to be a hotrod (all evidence to the contrary so far).  I've got some more planning to do, but I feel the front end redesign project is going to happen sooner, rather than later. 

Rusty