Brian,
We're rigged with low cars as you've described on the inside track and the springy things on the outer. When hard on the wind I use the inside tracks with the standard jib sheets. When sailing off the wind I run an auxiliary sheet through the outer track springy block to a ST winch on the cabin top. This opens up the slot a little and I think improves performance. It's straining at gnats for cruising, but it's one of the ways I amuse myself. It leaves Donna bemused, but she humors me. The cabin top block isn't ideal, but we only have one set of big winches in the cockpit. When off the wind the pressure on the sails, hardware, and lines isn't as great, so they seem to handle the loads.
Since we got the boat I couldn't get the sheaves on the low inner cars to spin and the line lead out weird, rubbing on part of the sheave frame. We purchased Garhauer low cars to replace the originals. If I'm going to install them, I need to do some tweaking. I won't be to the boat for several more weeks so can't give full details on this at the moment. I'll report on those developments. The build quality is great but there are some secondary features that aren't optimal.
If you're going to use low cars on the outer track, the only thing I'd say is to try to mock up the geometry to be sure the lines will lead without chafe or obstruction. Check this through the full range of possible car position.
Cheers,
Jeff