I look at it like the RB-5 and STR-4.
Both cars looked almost identical at launch, but were far from equal on the track. They both came from the same mind, built in the same shop.
The cars were so massively different on the track, how could anyone look at both cars and tell me which one would actually be quicker, without knowing of course the B team would be slower.?
I'm not doubting your aero knowledge would give you an edge on telling which car _might_ be better than another ringo, but when the entire field is separated by just a second (not counting new teams of course) in qualifying at some races and you can't see the floor for example, it's too tight a call for even human to know it can't win a race from a limited set of pics.
Maybe the chunky rear is there for a reason. Maybe there is aero trick in the diffuser we can't see that trumps the loss you perceive. You can't look at the RB6 and know that it has a trick front wing (or whatever it was) that can flex around the rules. You couldn't look at the MP4-25 and know it had the F-Duct at launch.
My point is, no one can 'know', but some can guess better then others.