Just another fat guy's opinion here...
I would say that a 32" waist is fine. 34" is touching. 36" is getting pretty snug. I think 39 to 40" isn't going to fit. If you can get down to 36", I'd say go for it. Could be a good incentive. For me, I need to get back down to a 34" before I'd say I fit. Body proportions, legs, etc., are different for different people so my opinion could be +/- 2" compared to you or others.
I would recommend you try to sit in a pair before you buy. Different seats also fit different people differently. For me, the GT3 seats are MORE comfortable than the stock ones. Good luck.