I've recently taken my old engine apart after 18 years and 30k miles. its an early 2 hole thermostat 2wd head and was 137.4mm when it went in and its still good.

there's lots of folk using low hieght heads with no problems. i reckon the likelyhood of any problems is more to do with a bad tune or overheating than anything else. that said, heads which are thicker command more money.