How much timing do stock C5s like? Also PE questions

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reason im asking is that i've tried to add 2 more degrees of timing today at the track and logged my runs. I gained nothing but I didnt see any KR either. My stock program gives me about 28 deg total timing at WOT, is this the pinacle of what the ls1 motor likes? Also, i tried playing w/ PE from 98-102% and saw little to no difference. That varied the o2 voltage from .860s all the way to .960s w/ little to no change in ET or MPH. Thanks dave