I believe that the compressor efficiency is not fixed per se - it is a function ultimately of both compressor pressure ratio and airflow.

There is typically a small "island" where compressor efficiency is maximized - probably around 65% at the most for twinscrews, then decreasing as pressure ratio increases or airflow increases.

As for measuring EGT, this is typically done to see what effect decreasing the AFR has, as adding more fuel and adjusting timing has physical limits (that is, adding too much fuel and retarding timing too much can cause your cats to melt). It's probably more detailed than you want to get into at this point.

Are there any good dyno places in Prince William?