I would say GO FOR IT. The engine in the NX650 and XR650L are almost identical. Somebody smarter than I can tell you the difference. For that matter there are a lot of similarities to the XR600 as well. I have looked into a carb change myself. There is a guy in Chicago (I think) who recently did a jet change on his stock carb and was very happy. To get maximum benefit, you would want to change the intake and exhaust as well, but then you are talking some serious money. As far as my research goes, I think the Mikuni Flat Slide 41MM
carb would be the best option for the money.
Keep us up on your progress.