I've gotten into the high 60s without cheese. I got a large PQ bonus on colonization (almost 50%), then a Precursor event later on changed the base class of the planet to 35. Apparently the 50% colonization bonus got reapplied to base 35, so my new class was in the 50s before improvements. Tack on habitat, soil, terraforming, and the Terraformer and you've got a really really nice planet.
The cheese tactic to get a really high PQ, which I've never used but which I've read about, is to build an enormous horde of colony ships, each with about 10 million people, and send them to the planet. Colonize, hoping for a worm/precursor artifact find, and pick the PQ bonus if you get it. Then destroy the colony. Recolonize, again picking a PQ bonus if you get it. The bonuses are cumulative--five +40% events yields a +200% PQ improvement. Repeat until you've got a satisfactory PQ.