Well I think it's on pair with masteron if you dose it at around 100-150 which is very expensive and masteron is deinifitely better for someone that has barely any fat on their bodies, same with proviron. Both anavar and winstrol are better than these two for building muscles and therefore better steroids, (more muscles, more metabolism). Winstrol would be the best cutter but a bit more harsh than var, winstrol is still a quite safe steroid and for some reason people think it's extremely bad for lipids which isn't true. Pretty much every oral at least bulkers like dbol, adrol etc will be harder on the body.
To answer your question, will it help? Sure but very little. To answer the other question, do you need to be lean for anavar? Nope because any steroid will have the same effect regardless of bodyfat. I wouldn't suggest a guy that has 300lbs of fat to run anything though because he wouldn't notice any difference on the body and arguably be in more danger of cholesterol BP etc, (he's gonna be stronger tho).. If you're 15% you might go down to 13, if you're 10% you might go down to 8. That's how steroids work.
Two guys at 150lbs one having 15% fat might need two cycles to get a full 8pack while the other guy at 10 only needs one. Granted diet, workout, genetics etc etc always plays a roll.