Based on the old adage that you should not run an image system that requires more than 2/3s of the mount rated capacity, your 7.5kg RC would need a mount rated at a capacity of 11.25kg.
Add in all the other stuff you would put on the scope, would you have a finderscope, a guide camera, imaging camera, dovetail mounts for the attachments, heaters, cables, etc and then work out the total capacity required.
I would imagine that your total weight with those extras might reach to over 10kg easy, which is putting you into the 15kg mount capacity range (using the 2/3rds rule).
Celestron published payload capcity for the Advanced GT VX CG5, is 35lbs, or 15.9kg.:
http://www.celestron.com/support/kno...tron+mounts%3F
You should be Ok, but check to se what other RC8 owners are using and why.