i have an P90 SG with a RWRP neck.
of course if you play the bridge and neck pickups seperately they sound just as normal.
playing neck and bridge together you do get a different tone becase of the RWRP neck - its noise cancelling obviously.
i'd personally consider this a benefit, particularly for jazz. the mixed position still sounds great, and because its noise cancelling it might get you out of some tricky situations live.
ok, maybe the mixed tone is then not an exact noiseless copy of what a normal neck pickup p90 guitar would have... but as long as it sounds good, does that really matter? in anycase i never A-Bed those two combinations.
when i play jazz, i personally prefer the sound of a mixed neck/bridge combination anyway.
if your playing with a overdriven amp, and you switch to the mixed position, and suddenly the hum is cut out, well maybe that would be off-putting to some listeners. but would you use the inbetween position with a dirty amp anyway?
finally just to say: i really prefer p90s with a RWRP neck, and when i have to use a P90 guitar without one, i do miss it.
i suppose some people might think differently about that ;)