Q.did i kill my pittosporums?
My pittosporums were quite large, 8-9 feet, so I cut them way back to around 2-1/2 feet. This was right before a heat wave that entered our area in mid June. The remaining leaves have all turned brown, and I have not seen any new growth (it has been 3 weeks). I have cut them back in a similar way in the past and they responded well. I am afraid I may have killed them this time. Is it a bad idea to cut pitts back so far?

Pittosporums generally do well with heavy pruning. The combined heat and pruning may have stressed the plant. A stressed plant can become vulnerable to insects or disease. Look for any signs of other issues on the plant and treat if you need to. Neem Oil is a good treatment.
I would wait and see.
Here are a few links for you.
https://www.gardeningknowhow.com/ornamental/shrubs/pittosporum/growing-japanese-pittosporum.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm