I've found that on my 1978 400A, I can hit and hold 70 without a problem. 75, I'm not so sure. I think that's probably close to the max speed for me/my bike as it currently stands... I weigh 185 and probably don't have it tuned as well as it could be. I don't hesitate to get on any roads that average 55-70, but on some of the faster interstates where people think you're slow at 70 mph, it makes me a little uncomfortable.
I'm sure the extra 50 ccs will give you a little better output, but I'm not sure how much.
I'm sure the extra 50 ccs will give you a little better output, but I'm not sure how much.