Re: MIM Precision bass?
If you change the electronics on a MIM Fender it won't "become" an American. The wood, hardware, and dimension specifications are slightly different between the MIM's and the MIA's. That is not to say that an upgraded MIM wouldn't make a great instrument. The first thing I would do to a MIM would be to change out the pots, caps, and pickup for higher quality components. My pots of choice are CTS and I really like orange drop caps. Depending on the music you play, Duncan makes P bass pickups like the Vintage, Hot, and Quarter Pund, among others. If you have soldered before, swapping out the electonics is not difficult at all, especially on a P. Depending on your budget, replacing the hardware might also be an option. Schaller makes great tuners for basses, as does Gotoh (for less, I might add). Replacing the bridge might also be a good idea, with Gotoh again making a great bridge for the money, and Hipshot making top quality stuff. These are all things you should be able to do yourself.
A very important thing is also having a properly cut nut. This affects everything from playability to tone to tuning stability. This is one thing that should be done by an experienced luthier.
At this point, you might looking at about the same amount of money that you would spend in buying a MIA P, but you won't have to spend it all at the same time, you'll get the parts you want, and you'll still end up with a killer instrument.