I have the 8.5 with a Thudbuster ST. The seat post diameter is 27.2 mm (it is marked on the stock post, although very faintly). The Thudbuster is a nice addition to the bike, but don't expect too much from it. It works best on dips, where the rear tire moves down away from the rider; the effect is negligible on upward bumps. I would recommend getting the Lizard Skin cover if you're going to be riding without fenders; both can be purchased direct from Cane Creek
Thanks everybody for the info. Yep, chepburn nailed it. New here and have been out of the "bike world" for about 20 years and didn't think about pulling the post and looking.
Thanks also to Deathly Hallows for you experiences with the ST. I'm hoping the LT may be a little more helpful with the upward bumps as you say.
Anybody have experiences with the LT?