champion leopard is what i throw for a straight thumber. it's what schwebbie uses too since my results aren't necessarily a great recommendation. (actually may be an old proline leopard but those are a bitch to get)
the more overstable a disc is, the slower it will turn in the air. if you want it to turn slowly and get more distance use something very overstable. i use a Z Predator(2.5) for all of my thumbers, i think its perfect. i have a Z Xtreme(3.0) ive tried to use but i think that it is too overstable for me to use.
when i want to throw a shorter thumber i use a less stable disc that will turn over faster like an avenger of flash.
i dont think a roc would be good at all for a thumber....at all.
I throw a thummer on a lot of holes. I use the Reaper for shots inside 250 and use the Flick for over that. Both are great disc reaper is a little slower and the flick comes out fast but can go a long way.
I've found (for me any way) the less stable the disc the longer it takes for it to complete the turn. For these types of throws I use a 150 flick, a champion side winder, or a road runner. The lighter the weight the easier it is to manipulate, but keep in mind that the less stable the disc the less reliable the flight path when throwing overhand shots. Hope this helps:)