That's really interesting about Jeff's bass. I always thought he'd have really low action and a straight neck because he is so fluid, and he really pulls off that legato stuff. And as you rightly point out the actual fret height makes a big difference too. Not quite sure where I stand on that; my 72 just had a fret job when the rods were done and there's not much fret left; feels nice, but unfortunately the bridge has run out of adjustment so I can't get the action quite as low as I normally would. That'll be a job for my luthier when I next get down (my normal luthier didn't do the rods, and unfortunately I needed the bass back before I could get the other stuff sorted). When that's done I'll let you know. I must confess from time to time I'm influenced by the "can't have it that low/straight" brigade and I try hiking my action a bit or putting more relief in, but every time it feels awful!
With regards to Jeff's strings, I've seen him say several times that he uses 40-100, although he may have changed. And my Alembic has pretty low action
(although that currently has 45s on it and I normally go for 40-95 Rotos on my Ricks). I actually once played a Jaydee bass with 30s on it and it had the most insane action I've ever seen, I couldn't even play it it was so low. And a fellow Alembic owner has an action of something like 1 millimetre at the 12th.....compared to him, my basses are like Jamerson's!