Well pretty much no new car has only what the brochure says - the other day I saw a video from some tuners saying the GTI's 245 is more like 265 on the dyno.... so by that token, I'd guess the R is putting out easily 20+ more than on paper - but then again I'll bet the M135i and A35 are too.
I wondered if it was aero drag too - but the A35 is lower coeffficient than the Mk8 R.
So not that.
Could be tyres, but didn't look like it could be unless both the BMW and the Merc had been to Kwikfit for some ChingPongWaos.
I can only assume that ultimately something about the way the R puts the power down is superior in this case?
From the paper spec:
A35 0-60 4.9s
M135i 0-60 4.6s
Mk8 R 0-60 4.7s
But what they were doing wasn't a 0-60 though was it? It was a standing quarter.