The salsa is made "small" (as narrow as mine, and about 6 inches longer) by using small batteries and also uses a smaller control system. Less amps. This doesent matter at 4 mph, as the lower gearing and speeds makes up for the smaller batteries. Add a 6mph option, and range drops by about 40 percent as the amps required to turn, move about even slowly, increase by the same 50 percent difference in speed.
This may still be OK if you dont go far. But will mean that average battery depletion levels go up by 50 percent too. And this means that they too will have a much shorter service life. All this stuff is why OTs are the wrong people to advise on power wheelchairs!!!
Now, skinny? Stll no problem, possibly. Fat, 6mph, small batteries? Recipe for short range and regular battery problems. Not that I am saying you are fat!