Two easy mods I did to my P-Nut.
1) replaced the just-a-Nut II with a brass just-a-Nut III. Easy to do, looks much better, and while it wasn't a huge change in tone, I like to think it sounds better to justify the cost (though the nut needed replaced either way)
2) filed the tailpiece to accept a non-tapered B string. Common issue with older Warwick tailpieces, but it got to be a pain uses pliers to squeeze the end and barely have the string fit. Now, with about ten minutes and a file, I can use any string and change strings much quicker.