Actually, stock on a US `00 M750 is supposed to be 41 teeth, but I wouldn't be suprised if some of them came with 39s. I had a 43 and went back to a 41, but when I did that I was also upgrading to magnesium wheels. So, now I've gone back to stock gear ratio and the bike still accerates better than ever, so I could probably even drop another tooth or two for some top speed, but I'm pretty happy with the ride right now.
Stock gear ratio (15/41) is about 2.73:1
15/39 would be about 2.6:1
15/43 is about 2.87:1
and 14/41 is about 2.93:1
I think you're saying that you're running a 14/39 which is about 2.79:1 so you're running somewhere between what stock for a 750 is supposed to be and my old 15/43 combo.
With stock carbs and flywheel, the 15/43 combo created a really nice low end, but with either the carbs or the flywheel it might be on the verge of too low and with both I was just wasting top end, which you don't really want to do with a 750 either.