I can't answer all your questions, but I can for some.
AFAK yes, you'll get a de-restricted map. (I have a 2023 Gt with Penner tune & mine is de-restricted) No, it won't erase cruise control, mine works as it should.
I think it does improve MPG, depends on how much flex you give your right wrist though. At one time I had to do many miles at a slow speed & my fuel burn was staggeringly good for a big bike like this. I was up to 55mpg (UK MPG too) I get less than that now as I don't have to do long slow rides, but still manage to get 300k's per tank no sweat & still have a bit left, like another 20-40k's sometimes.
Your MPG could be down / different because of the different "gallon" sizes. Metric is the same everywhere, just so much easier, but I guess it's what you're used to. I assume you're talking US gallons, but even so, 21mpg seems very poor TBH. I'd have the bike checked - or do it myself. make sure all the plugs are firing etc.