Jezmo,
I prefer to measure compressions using swept volume same as used for 4 stroke car engines, mainly because even a 4 stroke has it's inlet valve still open past BDC (gas inertia..blah blah blah). It's a bit of a controversial subject as to which way is more correct but because 2 strokes all have very similar timings then either way can be used. I just find it simpler using swept volume because I'll have already measured the stroke or found it in the manual.

With the Enya, starting was identical (one flick, can't improve on that ) but you can feel the difference in compression although it was still quite easy to flick over. From memory (a hard disc crash wiped out all the details) I increased it each run by .5 at a time until the rev curve started to flatten out then went by .25 for the last couple of runs. There was no difference in handling on any run. As far as tune goes, the main needle needed to be about a quarter turn further open but the idle surprised me because I had to lean it out considerably (the Enya uses an air bleed for idle but also has a midrange adjustment). Idling was much smoother and lower, I suspect because it had much more useable idle compression which made it more efficient at burning fuel. In flight it gave one more surprise in that it unloaded like crazy , far more than it did before. It's never burned out a plug and got quite a lot of flight time after the testing (which really didn't take all that long anyway). For interest's sake, the modded .60 exactly matched a standard Enya .80X on the same prop and fuel.

I had to machine the head to raise the comp to the level I needed because taking out any shims simply doesn't do much at all. As I said, I lost my data but I'm fairly certain I ended up lowering the head by .024", most shims are only about .008" and usually only one is fitted anyway. On the Enya, removing this much was no problem because it still had around .020" squish clearance. I've got a self imposed limit (meaning hey let's not take any chances here ) of .010" squish clearance.

Engines will vary as to how they respond though. I did a quick and dirty mod to an ST 45 not long ago and only saw a 200 rev increase which worked out to about 4% extra HP but I was basing the before revs on a run I did long ago, not back to back. My son bumped up the comp on his SC 1.08 and got a massive 800 rev rise. This was the same increase I had with the Enya but his engine was only turning the prop down around the 8500 range where the Enya was in the 12,000 range.