10R80 5th to 6th shift issue

[davbrucas]

Ever since I bought the car new it would hesitate to shift from 5th to 6th. It felt like it was slipping going into 6th. It would rev a bit despite no increase in throttle input and then shift. This was only at part throttle under some load. I recently installed a whipple and had it remote tuned by Lund. The part throttle in boost 5th to 6th shift is way worse now. It shifts fine while not under load and during WOT, however. My tuner said he had no control over that in the tune and said to work on the bypass valve to prevent full boost at part throttle and changing my driving habits. He also said he cannot tune the TCM separate from the PCM which seems wrong. Any opinions or suggestions? I will post of video of what it's doing once the rain stops.
Thanks in advance.

2021 GT premium

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[davbrucas]

The above is in sport+.

I drove it in track mode today and it is very sluggish off the line and it immediately shifts thru all the gears and into 10th by 40mph. Then it lugs and shifts down to 8th or 9th and back to 10th. Track mode is not usable. None of this was happening prior to the whipple and Lund tune. Very frustrating to say the least.