Hi made the same mods myself to MIM Precision and along with a tone cap mod (cornnel dubliner oil in paper 0.1 uf capacitor)
I opted for Semour Duncan Basslines SPB1 Vintage, because I wanted more old school thump (think Mowtown), I also intalled a Baddass 2 bridge.
Probably similar to other comments I can't say I noticed a massive shift in sustain, but they do allow you to file your own grooves in the bridge saddles so you can get your action real low (and they look cool).
Again there are better/cheaper more adjustable high mass bridges on the market, but as I haven't tried them I can't comment on how they would compare.
Spending money on decent strings is also a great idea.
If your looking for hotter pickups Semour Duncan do SPB2, and even hotter SPB3 "the quarter pounders".
All these pickups are resonable at around £40.
Incidently, Fender actually fit SPB1's to some of their more expensive signature basses (I think the Steve Harris bass has them)
Steve is Iron Maiden's bass player and a long time user of Semour Duncan pups.
You solder the Oil In Paper capacitor in the same place the old capacitor was soldered so no wirning diagrams are needed. (pop the old one in the bin)
So for not much money you can improve your MIM basses tone and playing action, pick up some new tech skills and have fun to boot.
If you have trouble tracking down the Cornnel Dubliner OIP capacitor, there are some Russian versions on e-bay, or you could use a Sprague orange drop capacitor 0.1 uf poly capacitor. All will be better than the cheap original.
You may also want to check your pots, I believe that MIM basses use CTS pots now just like their American cousins. If your bass is early it may not.
There are much better bass pick ups out there, but I wouldn't fit them to my MIM Precision....only because I don't love it that much and you wouldn't get your money back if you sold the bass with the expensive pups still fitted.