IMO Harden was a key factor in OKC's series win over the Spurs, and they might miss him if they meet the Spurs in the WCF again. However, Harden didn't (and wouldn't even now) make a great difference vs the Heat. The Heat are built perfectly to counter perimeter-oriented teams with James, Wade, Battier, Chalmers, Allen. I don't believe KD/Westbrook/Harden/Sefolosha would have been enough, and neither will Carmelo/JKidd/JR Smith/Felton/Shumpert.

The Heat are vulnerable to size + scoring ability in the frontcourt, and it's no surprise that the Pacers with West/Hibbert/George have given them the most trouble. Which is also why I agree with the general sentiment that the Spurs with TD/Splitter/Diaw and LAC with Blake/DJ/Odom should be able to give the Heat a better series than OKC can.

So OKC losing Harden is not going to cost them a le per se. What OKC should have done is to amnesty Perkins and trade Harden for a solid big man who can score.
OKC's core is alot younger than Miami's core. Eventually the Heat would get older possibly allowing OKC to surpass them. It won't happen now with just Durant and Westbrook.