The 3XV is a nightmare to derestrict, and unless you (or someone you know well) are in Japan, getting the parts could prove extremely difficult.
To get 'real' power, you MUST get an SP, the 'R' model has a much smaller exhaust port (plus it's the wrong shape!), and the list of go-faster goodies is really a TZ spare-parts list - ignition, heads, etc.!!
Pipes are available through RG500.com in the USA (please go through our Links page to get to them), and they will ship to the UK, but without the ignition mod's and other TZ parts, you'll be hitting the low 50's only. (Some similar restriction problems as the MC28, but an HRC ignition card is readily available for the Honda - for now!)
Once tuned and set up though, capable of superb power up in the mid 70's. The 3XV-SP Matt Patterson from TYGA Performance used to race was 76.something hp! Don't expect to get that without a lot of money though.
In terms of V.F.M. though, the Yamaha can't even compare to an MC21 I'm afraid. Delimiting the NSR's ignition is free, the porting on the 'R' and 'SP' is the same, the suspension is typical Honda quality (maybe change/upgrade the oil), and the build quality of the NSR is superb.
Reliability on any 2-stroke is down to how it's (been) looked after I think!
Andy.
NSR-WORLD.COM