I have two unrelated anecdotes to share, one involving a B&M kit, and the other involving TransGo, both on different daily-driver transmissions, neither of which were GM. So take the following with a nice big grain of salt.
The B&M kit was in a Ford 4R70W, on the bench for a full rebuild on account of a failed forward clutch piston seal. Upon teardown, I discovered that the intermediate clutch friction teeth had been hammered so hard that the teeth were worn down to half of their normal width. Hard to say how much longer they would have lasted. I removed the B&M springs and reassembled the transmission according to mild J-mod specs. In my opinion, this made the shifts feel satisfyingly firm, yet not harsh.
Later, I was rebuilding a neighbor's unmodified Chrysler 41TE. IIRC, it was showing early signs of slippage. The owner requested a shift kit, so I did some hunting around and found a TransGo kit. I found the installation instructions to be very user-friendly and confidence-inspiring. After doing the whole calibration procedure thing, I have to admit- I couldn't even tell that a shift kit had been installed. Whatever it changed, felt very natural.
It's completely unfair to compare these two stories against each other, but FWIW this is how B&M left a bad taste in my mouth while I wouldn't hesitate to go with TransGo again.