Well this is an interesting thread...
I think you're misunderstanding what a spoiler does, it's not creating down-force... it's adjusting the aerodynamic profile of the rear to minimize/eliminate lift. That doesn't increase drag like a down-force generating wing would. The front splitter is moving air around the car and creating lower pressure underneath the car, helping create down-force. Whether this creates extra drag, and how much, depends on how the rest of the car handles air moving around it. I think the Camaro is just less aerodynamic because of it's shape and extra cooling abilities, that creates more aerodynamic drag. The wider tires hurt rolling resistance as well.
Those tires were another huge difference in this comparison. However, tires are nothing if the chassis can't locate them properly... they're useless if the suspension components can't keep them on the ground and pointing in the right direction.
I think the Mustang chassis (that's platform and suspension components) just aren't advanced enough to provide the level of performance an Alpha platform car was designed for. You have to understand that the Mustang is bespoke for Ford and they have to be able to sell the the thing from a $20k EcoBoost grocery-getter, all the way up to the GT500. GM is going after Euro luxury/performance cars with Cadillac, and they spent A LOT of money to develop a platform that will get them there. Luckily for us they also decided to base the Camaro on that platform. The CTS-V (closest relative) is meant to go up against M5s, E63s and Panamera Turbos... cars that cost over $100,000 in some cases. Anyone who's seen the underside of the Camaro knows just how much aluminum is in the front and how advanced it all is.
I think GM saved money as well, that's especially evident in the rear suspension... the car could have been even lighter if they used aluminum for the rear subframe and control arms. But judging by the lap times, spending so much $ on the rear-end would bring diminished returns. It just has to be able to not flex like a noodle under power.
There's a youtube car review channel I watch, the guy does great reviews... I've linked the underbody sections of the GT, GT350 and ZL1 1LE (same front end as the SS, except for the DSSVs, unlike the Mustangs). Looking at the front end will should you exactly why the Camaro is good.
(edit: looks like time stamped links are wonky... start at
04:12)
(edit:
05:00)
(edit:
14:16)
I really like the GT350, R and regular. I would have bought one if they weren't marked up to the moon. As time goes on though, I realize more and more that the Camaro is built on a better platform. Lap times are just evidence of that.
Does anyone know if PP2 will feature steering knuckles from the GT350? It's actually where I think Ford is going wrong. Once they developed the GT350, they should have given all the new components to the PP2 GT and taken away the more expensive components as you went down the trim levels. People bough the Shelbys for the name and engine, most of the current owners wouldn't have cared those chassis components were made available on the GT. They wouldn't have bought a GT anyway.