Max out a 15/13/15 (a/d/s) RS/EQ Rhydon?

I have a similar one that I've just taken to 3000 CP. Depends on your local area, I never use mine on attack because it's too slow but if you need another high-CP mon then go for it.

He is a decent counter to Tyranitar so yes.

I would IF you need another high gym placement Pokémon but with the news of gym rework, I'd probably wait. Don't fret however. It'd still evolve into Rhyperior in Gen 4 provided it will come before the game dies or if you're still playing by then. If your intentions are just for Tyranitar or Blissey counter, I won't touch him no matter how good the IVs are. There are better Pokémons to choose from.

That's 96 IV right? At level 30 it will be at 280X CP so I think if that is already sufficient for any 'entry level' gym.

Power up further if you are really desperate to get into gyms by CP.

As an attacker it is not good.

I am holding out hope for TM's as I have a similar Rhydon.

As far as powering it up, with all the talk of gym changes, I am saving my dust. If there is 1 species limit then a high cp pokemon like Rydon, will see more gym usage. The one species per may not be the only change, there may be a first in top, last in bottom system also but into place. Then flawed pokemon like Rhydon with major weaknesses, but high cp will be less useful.

I wouldn't max it, but it's not a bad Blissey counter as is. Little slow, but it's got the bulk to weather the storm.