Tony, its all a case of application really.

As mike says if you run high compression and big boost then you are going to have MASSIVELY retarded ignition, and thats going to lead to an excess of heat in the exhaust side of the head and manifold and turbo, not good for reliability obvioulsy.

The upside to high comp is that it will be marginally more economical and slightly more powerful off boost, but who the hell drives a cossie around much while its in vacuum anyway?

For a high milage road car on normal road fuel, i would go std comp or lower, i wouldnt even consider high comp personally in your circumstances, those 7.5:1 pistons you mention are probably a good bet
Talk to stu though, if he is the one mapping it its his opinion that really counts.