To those of you who are using the karma urethane, may I know what surface do you guys use it on? I find the OOB quite strong.

I am rev dominant, currently on OOB, using it on an extremely light THS with bone dry heads. First game is always good as I can hit the pocket easily. But when the mids go, I struggle to find a line to the pocket because the ball starts hooking too early midlane. I tried moving deeper, but it would either burn too early, or even if it does make it downlane it would lack energy to turn back to the pocket.

Should I bring the surface to 2000 or 4000?

Taking the surface up to 4000 should help this. You will have to move right on the fresh but wont have to move as much later.

Took it up to 4000 and there was considerably more length but breakpoint shape stayed the same. Should be good on a normal THS.
Unfortunately at the same time, my center changed the pattern to a ridiculous 30ft THS and I was throwing from gutter to gutter.

Lets say if I polish it further with secret sauce, what changes can I expect?

More length, less overall hook.

Urethanes react very differently from resin balls, which when you polish them, you usually get more of the hook transferred to the backend.
 
The only time you'll get more backend from a urethane is if you slow down your ball speed or you encounter the kind of oil pattern that gives it to you (long-ish, light oil and very clean backends). Short oil tends to make a urethane roll out and/or hit weakly because they tend to grab the lane much earlier than resins. Too much dry is the enemy of most urethane.