It's an interesting one. I'd agree that these days there's little to choose between the physical build quality of the MIA, MIJ and MIM Fenders, even some of the top end Squiers I've seen recently aren't far off....better computer controlled machining maybe
The main difference seems to be the relative weakness of the MIM pick-ups.
I picked up a new MIM sunburst JB in 2006, swapped out the bridge for a Badass III, the white scratchplate for a Fender replacement tortoiseshell one (for that 62 reissue look) and a pair of classic Seymour Duncans and still saved myself £300 off the USA model. The Badass really helps with getting the action down, general intonation and definitely gives a beefier sound (as you'll know from the Geddy), I'm not sure why Fender don't fit them as standard. The SDs are just miles better than the standard pups.
I also had the action lowered, neck adjusted slightly and intonation set-up by a local luthier, which has resulted in a very playable bass.
Previous to this I have used a mixture of a 60s Epihone Rivoli (which I really regret selling), a couple of Yamaha BBs and an Ibanez Roadster (all long gone) but no American Fenders.
I also have a Westone Thunder 1A which I'm working on at the moment, great bass with a really punchy 80's sound and if I need a workout I can just use that, as it weighs a ton!
Personally, if you have the time and inclination, I'd find a well built MIM or MIJ Fender and spec it up (which the Geddy Lee more or less is) but having only played the MIA models briefly I may be missing something.