The standard sprockets on our bike are 16 front, 45 rear.  I felt this gave it too low a gearing, so just swapped the front for an 18 tooth.  I'm definitely liking it - no problems pulling away in 1st, 2nd has a more usable range (no more going round roundabouts in 3rd), and 5th is still good at an (actual) 30mph.  The engine doesn't feel strained or struggling, and it makes good use of the low down torque that we enjoy.

One thing it hasn't done is increase the top speed.  I'm still topping out at an (actual) 65mph.  I suspect that's down to the convoluted cat exhaust strangling the engine, but will try up-jetting first before I start hacking up the exhaust.

I bought this sprocket (UK seller) listed as for a "1995-1997" Virago, type "569" (c.f. JT569) but with 18 teeth (+2).  It fits just fine, and there was plenty of slack in the chain to get the larger sprocket fitted.

Something to note is that it's a 520 sprocket, as on the original Yahama. For some reason, the stock sprockets and chain on our bikes are 530 (same pitch, but wider). However, a 520 sprocket is fine with a 530 chain, as long as you stay on top of the alignment, and I'll be changing the chain and rear to 520 anyway when I replace them.