that is really getting pretty big, really big.
there is a guy in Italy using an EQ8 and RC 16" I haven't heard of anyone going any larger with the EQ8, perhaps you could get away with an 18" newt (with its lesser FL) but yes I think its heading to the serious mount territory... at 20"
TS has it on sale for about 5000 euro more than the 16"
http://www.teleskop-express.de/shop/...russ-tube.html