Even the stock NT carbs are very reliable and can perform very well if tuned properly, it all depends on what other mods you're doing to your engine, if you are keeping the engine basically stock with maybe the addition of a better exhaust, better NGK plug, better plug wire and maybe a better air filter, IMO you can due just fine with the stock NT or you could go up one notch and get a Speed Carb. from Pirate Cycles or from BGF on ebay, I dont have a speed carb. but others have said it offers a slight power increase when tuned properly so that could also be an option to take things up a notch.
Just remember that any carb you get will have to be tuned to the engine and the fuel mix you are running, if you are running Opti 2 @ 100:1 and have the carb tuned perfect for that mix and then decide to mix up some mix at 32:1 with conventional oil you could cook that engine because it will likely be way to lean from what I understand, so I would personally advise that you get the NT tuned real nice and run it for a while and see if it serves your needs because for most MB owners it is plenty good enough for their bikes and the speeds they are shooting for, I have the NT's on all my engines and I can get 35-39 MPH out of all my bikes with them and none of mine are heavily modified.
Hope this helps...and if I'm wrong on something here I trust that someone will step in and make correction, I can only speak from my experiences.
Peace