Might want to read the A10 recall in the TSB section. Hard shifting is a symptom. And yes it will effect stock cars just as well as modified, rather TCM tuned or not. Not saying this is your issue but your car falls within the recall range as does mine. And you'll want to read the whole thread for all relevant details don't just skip past posts. GM is refusing to fix this issue, and in our case they already blamed
my engine modifications for causing
your issue just as well as mine.
https://www.camaro6.com/forums/showthread.php?t=632693