Regarding shockwave, you are correct.. expertise cap, hit cap and spell hit cap all do effect it. At 0 expertise, shockwave is going to be slightly better than devastate
I believe revenge stun does not count for diminishing returns. And yes when its useful, thats when you spec imp revenge.
However, as Ive been reminded, this is not a thread about usefullness of stuns or stamina or anything else. Its about single target threat. At high levels of gear, revenge is no longer part of the high rage rotation. It has insanely high threat per rage so it does have its uses.
Btw playing with various stats, which abilities are higher threat seems to vary quite a bit based on hit, expertise, attack power, set bonuses, weapon speed and talents. I would recommend running the numbers yourself on a log, unfortunately there is not a good website at this time for parsing world of logs for threat and no addons that I know of. I dont think it would be very difficult to make them, but im hoping someone will feel the urge to rectify the situation so I won't have to