It’s unfortunate that the US market apparently isn’t big enough to warrant a more powerful version of the 350 - with the mild state of tune the engine is in I can’t think that 5-8 more HP wouldn’t be hard for the factory to come by without changing the sweet nature of the motor.
You might want to wait for the 500cc version of the "J" OHC engine, (which our local RENA guy told me in late summer 2021 is already in development). I expect this be just what you are asking for, power-wise and with very little penalty in fuel economy.
I think that development of the 500 model will be very easy for RE to keep a lid on, the machines will likely look identical to production 350s. They'll want to keep a lid on it too, because once out, it's almost sure to GUT US sales of 350s, kind of like what happened to the 500 UCEs in the US when the 650s came out. There's just not not enough space between the ~$4600 dealers are asking for the 350s and the ~$6200 they want for a 650, to position the 500 so that price will factor in. It's hard to imagine that a US rider is going to be convinced to downsize for $800.
Maybe when gas gets to $10/gallon, RE will start publishing fuel efficiency on their specification document. In India it's right at the top of page one. This is the one area where the RE long stroke single really shines, and the USA just doesn't care.