I have had 2 vmj's in the past and both were nice sounding ,especially the natural one with maple body,
i had read there would be a big improvement by replacing the duncan designed with u.s. seymour duncans there wasnt ...
i used the vintage sjb 1's as i wanted more of a classic fender sound but most people couldnt tell the difference... i also replaced the electrics with all fender original and this also had next to no audible effect
my conclusion was that they are very good as they come and i would leave as is unless your after a bigger output in which case the quarter pounders would give you that, but there is always a sacrifice made with that classic jazz bass tone i.m.o.