Yeah although I can't say 100% I'm fairly sure once you break the quality tier it has no bearing on increasing your chance to HQ by adding more quality. So essentially you can stop around the 270-300 range and finish the synth. That should save some time.

I've literally synthed hundreds of 300+ quality synths and have seen no difference in the +3 rate. Moon cycle, facing east, hopping on one foot and all that jazz don't effect it either.