The premis that only the high end systems allow you to adapt to factory sensors is incorrect. In fact the cheapest system - Megasquirt - supports this, and tons of people take advantage of it.

MAP vs MAF is not a big deal. It comes down to tuning. When you tune the ECU you essentially define the volumetric efficiency of the engine at different operating conditions (MAP vs. RPM). Once you get this map properly defined it will run great. If you have a MAF you now have a method to know explicitly how much air is entering, as long as you choose an ECU that allows you to use a MAF then tuning should be easier for you. I would expect the MAP guy to spend more on dyno time and the MAF guy to spend more on hardware and/or ECU development time.

All this talk has me looking through the garage for that old Digifant system I have laying around from a past project....