A 150 tire wont pop of a stock rim. It will just cause the handling to be worse, and less rubber on the road because of the profile.

Yes, a narrower tire is more flickable than a bigger one, for this very reason, i only run a 110 front on my 17s.
But saying a smaller tire on a 21" rim is better than 17s on sm rims is BS.
It doesnt help if theres no rubber on the side of the tire after you "flicked" the bike

As i said, the 17" front makes the bike much more flickable, and the front tires are usually only 110 or 120 in width, with a much smaller profile, usually around 60-70. With a smaller diameter comes less centrifugal force = better steering. With the smaller front, you have more weight towards the front aswell. It will handle way better than a 21" rim with a 90/90 street-tire. It really sounds like you havent tried this...

Racebike dont run narrow tires, or bigger diamteer rims. A YZF R1 runs a 190 tire in the rear. But surely they would want more topspeed and better handling. Im sorry, but I think you'll be laughed at for saying that at a racetrack.

I get better milage with the Sm wheels, as the whole bike sits lower, and have less drag. But economy shoyld get you into this, a new set of rims is pretty expensive. So if you run smaller diameter tires, to save on gas, this isnt for you.

Sm bikes emerged in the 70's in an attempt to get streetriders, dirtriders and MX guys to compete against each other. They were actually called Superbikers back then.

Anyway, to sum it up:

A 110/70x17 will outhandle, and outride, a 90/90x21 on the street any day of the week. Theres no point in talking back and forth, if you really disagree on this.