Ving.
There's not much difference between 8 and 9.25 inches so there's really no point stressing the aperture difference. In terms of light grasp, I used to have a 8" Skywatcher newt and it was hard to tell the diff on planets like Jupiter and Saturn in terms of brightness
Jupiter was very bright in the 8" with a 3x, so I'd imagine it would still hold up with a 5x.
You'd need a little more gain, but Registax will fix the graininess.
Like anything, see if you can try one before you buy one. Otherwise stick to a 3x...or even 4x barlow.
Jupiter is so bright, it shouldn't be an issue.