Most roads out of Margaret River would be 110kmh. Might be best to transport bike to a quieter road then ride.
Probably does not need to be said but Caves Road is maybe best avoided due to idiots in cars who are visiting wineries (not all are idiots but enough are to make it dodgy) with a combination of free wine tastings and confusion about which winery to visit next. When we had a place down there, we pulled cars out of ditches more than once because the driver had not been paying attention.
The Gracetown road might be an interesting ride. Nice beach at Gracetown (but bloody cold) and some good lookouts along the road.
There could be some interesting rural roads around Cowaramup.
If you're taking the MTB, Boranup Forest could have some track/dirt roads.
Maybe talk to these guys at the bike shop in Margaret River:
http://www.margaretriver.com/accom_result1.asp?Code=7756
And then there is the bike frame builder down that way. He could be worth a visit if it is raining and not real good riding weather. I don't know whether he takes visitors kindly or not:
http://www.pavebikes.com/
SteveA