I've got a Deore M591 rear mech on my RockHopper. It's a 9 speed mech but I'm running 8 speed. The shifting is pretty much faultless: quick, precise... Remember, there are many years of Shimano trickle-down development between your old LX derailleur and a modern Deore one, so I think you would find modern Deore at least as good as old LX.