Wow. That's a solid niche. I never see the camaro in dt toronto. Lots of mustangs and the odd challenger though. Personally I don't quite get the appeal of these cars. It's not so much the big engine format, it's just that they all appear to be so unnecessarily big. It's like they are not even trying to show any physical restraint. The challenger in particular is massive. The a5 is 6-9 inches shorter in Length and at least a couple inches lower and they still somehow wedged a v8 under it's hood. Why would Kia want to emulate the pony car?
I think Kia is only trying to make a "halo" car, however by all accounts, before they do, they need to hire away a suspension engineer from BMW or wherever. Visual design won't be enough.
I'm getting off topic but in response to the comment about dimensions, here's a comparison. The answer I think is simple, Audi is a more comprehensive overall design making the most of space, whereas the others don't bother minimizing overhang or eliminate under the hood extra access space for example. By the way, you mean the S5 which is the same body as the A5. The A5 only has a 4 cyl. 2.0T this year, and last year had a 3.2 V6 as the biggest engine.
Audi S5 V8 Challenger V8 Mustang V8 Camaro V8
Wheel Base 108.3" 116.0" 107.1" 112.3"
Overall Length 182.1" 197.7" <eek!> 188.2" 190.4"
Weight 3,858 lbs. 4,170 lbs. (OMG!) 3,605 lbs. 3,860 lbs.
I should add that I find the Challenger the best looking, and it has a great sound too. But with those numbers, its a No from me, and thank you very much for coming. 
As always, there are trade offs. Both GM and Chrysler only had a full size sedan to build their cars off of. So, it is no big surprise that they are bigger and heavier than the others. All of them will be bigger, cause they have very torquey, big V8's. And unless you put a bunch of money into expensive components, pieces strong enough to handle more than 400 lb/ft of torque will be heavy. While the Camaro is just a pretty silly design, the Challenger does give you a real back seat and a huge trunk.
Lets not forget that a loaded S5 is pushing $75K, while the Challenger just squeezes over $50K. And the Mustangs and Camaros (GT500's and ZL1's) can get up there, they are also WAY faster and much more powerful. And still are a lot less money than the S5.
I can't imagine too many people cross shopping a Challenger SRT8 392 with an Audi S5. Could happen I guess, but the $20K price difference will keep most away.
Not really fair comparisons, cause they are vastly different cars, designed to appeal to different drivers and offering a different type of performance.
