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bclark89 , Welcome to the place and enjoy your stay.
To answer your question...simply put NO. Cold air entering the exhaust tailpipe and working it's way up to the engine and bending valves , damaging springs etc is total rubbish.
To start if it IS cold air it is going to sink not rise. The warmer air in the exhaust system will tend to stay there until it cools , then drops. Any other air will be warmed as it progresses along the exhaust system in any case.
Back pressure is the result of exhaust systems built to a cost factor not a design. Most car makers fit whatever is cheapest to achieve their goals of weight , noise reduction and packaging dimensions. Performance is a low consideration. Even warranty claims rate higher for them !
Whatever GM fitted as standard would have been well tested before it was bolted in there.
Cheers , Pete.
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