I have had a Revo remap twice, once on an A3 and once on an A6, I wasn't that impressed with either.
I had an Emaps remap on our 123d and that was impressive, and a DMS map on our 535d touring that was brilliant, massive difference.
However, the most impressive tuning I have ever had was the D6 on my old E320cdi and the CRD-T+ that is on our current X3, both are tuning boxes and both sit on the turbo as well as the fuel rail.
Not saying don't go for a remap, but I would hunt our a Brabus if the right price as they are very impressive on you know what you are getting with them too.