So I got the LePree a few days ago, today I bench-tested it with a Shimano 7-speed indexed shifter with the cable attached to the derailleur in the normal position just as a baseline test, and...immediately it looks like I might be in luck! The movement is close to 5mm per shift. Next I'll unbolt my current RD from my MTB and try the LePree on there to confirm that it lines up with the sprockets and can reach 1st and 8th - it looks like every last bit of the movement range will be needed to reach all 8 sprockets. If that looks good, the next step will be to run the chain through it and test it with the bench-rig 7spd shifter. Finally if that works well, it will be time to commit and put the LePree on the bike properly to see how it runs with the 8spd shifter.