Blowdown would not change as all ports were raised 1.5mm (the same amount) by the 1.5mm spacer although, come to think of it, since the purpose of the spacer was to expose more of the exhaust port (as the piston covers quite a bit of it at BDC), the spacer may actually have increased blowdown. I removed 1.5mm or thereabouts from the top of the cylinder. Just using stock head so squish would be the same assuming I took off the perfect amount from the top of the cylinder - which I probably didn't. If anything, you err on not taking off enough so you don't have any issue with the piston hitting the head - the minor loss of compression is actually helpful in fighting high engine temperatures. so if you lose 10 or 15 psi, no big deal.
You can always shave the head down to regain that lost compression if it's keeping you awake at night or take a little more off the top of the cylinder.
Anyhow, I haven't even tried to widen the exhaust port yet. I found a machinist who charges me $20 a cylinder to take material off the top of the cylinder. Beats the hell out of spending $200 on a bigger kit and matching head and I could probably get it to a top speed similar to that $200 kit. (edited)