Every motor is a little different. In my opinion +4 is good and safe "starting point". I have read, that only few motors didn´t like anything over +2 or +3.
I have basic bolt on´s with home made porting. I have custom cut domes, 9mm band, 20cc and 1.15mm squish. 155-160psi. With our "93" pump gas, I have been up to +8.5 on timing without any issues. However I didn´t feel any "gains" after +7.5.
I felt a huge gain going from +5 to +7. Mostly I have been around +7 with T5´s and A&S Drag pipes.
..oh - and this is from sea level up to ~500ft above.