This is what I found on B&M Synthetic Trick Shift ATF
Meets or exceeds Mercon or Dexron III requirements
Your trans specifies a fluid that meets or exceeds ATF+4.
You only added a quart, so what is that? 10% of all the fluid? Yea, likely its NOT going to create some huge change.
And firming up shifts changing the electronic controls, I think, would NOT harm anything, probably reduce wear on the clutch plates. Really firm shifts might increase shock on the rest of your drivetrain, but again, I'd think it would have to be extreme to start any earlier wear on joints and such.
The only way I can think, that a fluid could increase firmness of shifts is too increase the overall viscosity of the fluid, so fluid pressure builds faster and higher in the system. Can that cause problems? Again, I don't know.
Please let us know how things turn out, after a couple of 10's of thousands of miles. If will be good to know, so we can warn people away, or re-assure them there is nothing to worry about.
One of the things that Bias
me, is the 41TE trans Chrysler used in its FWD vehicles, and especially the heavier mini-vans. This trans was very troublesome, and failed early all the time, using anything but the exact specified fluid would result in a quick death for that trans. Chrysler developed ATF+4 as a super high quality, severe duty, fluid that got that trans to last much longer. I guess Chrysler was buying the fluid in bulk enough, that they decided it was economical to use in all their trans, and being a better fluid would be better for the trans also.
So arguably, only the troublesome 41TE is the only trans so delicate that you would be foolish to try to experiment a bit with it and try to change things, but having a vehicle with this trans and hearing all the stories, I get jittery thinking about any fluid other ATF+4.