Rain mode on the V85TT does not affect power output. It just reduces the non-linear throttle control programming a bit, apparently makes ABS and Traction Control a bit more sensitive (Iíve yet to feel those effects and doubt you would on a dry road) and per the owners manual restricts top speed. I havenít had opportunity to check how maximum speed is limited, but regardless power at any lower speed is completely unaffected and I believe Iíve probably had it to 90 mph or so in Rain Mode without feeling any effect. If you wanted to go flat out, for example on a European highway, you could switch to Road Mode and then back again easily enough.
I fooled around with all three 2020 modes yesterday while blasting along on a local twisty road. Rain mode seems the most pleasant to me and ultimately on that kind of road Iíd choose it to make the best time, most easily.
I think the manufacturer screwed up the TBW programming on the 2020 V85TT by trying to make the engine feel extra zoomy, as you open the throttle it gains sensitivity with rising RPM in any of the modes. The result in Road Mode feels gimmicky and contrived to me, slightly notchy and anxious on a bike that is otherwise very settled and predictable. I think Rain Mode feels considerably better, with less of this effect, and although I havenít ridden one Iíd guess the 2021 model with its revised and extra modes may have addressed the issue.