Layla - I've been playing guitar about 35 years and took up mandolin about 3 years ago. I think part of my problem has been that I've always approached the guitar as a means to writing rather than as a musician would. As a result I don't have the proficiency that 35 years worth of playing should have provided.
I only did limited hand drumming, but I would think it's advantageous in learning to play any stringed instrument or keyboard. The thing I've noticed in friends that play tabla is their wriist and hand strength. They have no problem in playing clean barre chords on guitar right off.
As far as practice, I spend about 1 hour per day on practice such as scales, runs, etc. and then about 2 hours rehearsing orginal material and covers. Then, I spend about 3 hours per day concentrating on writng. Currently I'm doing a lot of transcription work to pay the bills, so my time spent on any one activity has a way of overlapping into another. I write and play a lot of bluegrass and fiddle tunes, so the speed is one of the few things I've got fairly nailed. Too bad there isn't more of a market for those kinds of songs.