Trying to correct the squish with a thinner gasket does not work because the problem is the match of the head to the piston and lack of consistant clearance across the squish band. The thinner gasket will add compression and reduce the clearance, but the effective squish is a function of both the clearance and match to the piston. For example, you could have a decent clearance of 1.5mm near the cyl wall that rapidly tapers off to 3mm+ at the dome, so you see little gain. Thats why the head must be cut.
If your not going to do the head, just drop in a .3mm or .5mm and be done with it, it doesn't make that much difference. The stock squish is sloppy enough not to worry about it. I dropped .5mm from stock(.3mm and .5mm) on my '03 250 and still had close to 2mm at the cyl wall.
I think the colors may have changed, but in my kit, .5mm is grey, .3mm green, and .15mm light brown and obviously the thinnest.
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