There's only one video of a TT/RS almost reaching the magic 300km/h mark but on watching it I got to thinking which would be quickest to this point if the RS4 had it's limiter removed. I can also only find one video of an RS4 reaching 300km/h and to me it appears to gain it's speed a little more slowly.
Until some proper test figures appear for the TT/RS this opinion is only one of speculation but I believe Audi are being as conservative with the TT/RS output figures as they are with the ones for the B8 S4.