From experience the best thing I did to aid towing wasn't lifting, lowering or moving the suspension or ride height, it was fitting HD springs all round. Improved the ride overall, effectively lifted (when empty) by about ~40mm 'cos there is now no droop from standard softish springs, but when loaded and towing the caravan it sits at normal height, nice and level.
One issue we still had was the tow plate bottoming out on 'speed humps' . that's been solved by grinding off the lowest run of holes giving us 2" more and no issues since. I'd guess that lowering the springs would mean the tow hitch won't pull at the right height, which is pretty crucial for a safe, steady load.
I've never heard of anyone actually lowering the springs on a Disco ..
My recommendation is not to do anything to start with, try towing with no mods at all, you migh be surprised how good the Disco is! If you need to do anything get Heavy Duty springs fitted, or spring assisters, like 2" plastic bungs you put in between the coils.
Lowering the suspension might bring more grief than it's worth.