150/90/15 is the largest m/c tire you can get.  140/90/15 will fit with little or no modification, a 130/90/15 is probably the best fit for the rim and basically the same height as the stock 140/80/15.

If you install 12.5" Intruder shocks, the sky's the limit  8)

That's what Oldfeller shoulda done for his 165r15 VW tire  

A little slower on take-off until you get the clutch engaged and rpm's hit about 2200, then can't tell any difference from the "seat of your pants" feel.  As I said, one drawback is first gear.  It idles in first 15% faster than stock gearing, so in really slow moving traffic, like a traffic jam, that can result in lots of clutch slipping.