From my limited experience so far (I just started practicing Moto Gymkhana recently), and also from what I have read from others, let's just say you had better buy rear pads in bulk if you are going to practice frequently. I would suggest that if you don't already, learn how to service the brakes yourself. It will save you time and money.
After last night, my rear pads are done, time to replace again (not even a month old).
The GS550 will be ok for Moto Gymkhana. Pretty much any two wheeled motorized vehicle that has a rear break and a throttle can do Moto Gymkhana.
People are using scooters, cruisers, sport bikes, trials bikes, supermoto, etc to practice/compete in Moto Gymkhana.
Regarding the use of clutch during Moto Gymkhana , I found this forum post by motogymkhanaman (aka Duncan MacKillop. Chairman of the Moto Gymkhana Association), on the AMGRASS forum, quite informative.
http://amgrass.com/forum/practice-practice-practice/practice-techniques-clutch/
I will be getting out at least a couple evenings next week. PM me if either of you are interested and we can arrange a time and place.