Well I've pimped my Tony Levin to within an inch of its life, and ended up with a bass that is [i]seriously[/i] superb.
However that came at a cost. I had a brass nut, Musicman battery box, a Nordstrand 5.2 pickup and a John East, 3 band EQ fitted costing in excess of £400........yes.....I know, but I had my reasons.
The East pre was an amazing improvement with the original pickup, but the addition of the Nordy has made it as good as any Stingray I ever heard, and I've heard/owned a few.
If you go down the pimping route it'll be worse for you because you have two pickups to replace, so make sure you really like the bass in the first place, because you'll never recover the outlay if you sell it on. In my case mine was a gift from the Mrs, so it's a keeper anyway, and I wasn't worried about tarting it up.
I'm down to two basses now, the Kinal which is my main bass, and the OLP, which is just as great to play, has a different but no less impressive range of tones, and was well worth the effort.