The question is can you buy a bay trail device that performs better than the best A7/S800 device and the answer is categorically no. Bay Trail can claim to be as efficient as the sun but if it's never exhibited in that capacity then the point is moot.
You make the assumption that cherry trail will be competitive in GPU, do you think apple is doing nothing with the A8 and Samsung with the S805? Intel will need to more than double it's current GPU while simultaneously maintaining or lowering overall power usage and increasing CPU performance. They've never done that before.
The S805 won't even a miniature candle to Cherry Trail, either CPU or GPU wise (the S805 is GPU focused and only 40% higher performing).
The A8 is a complete unknown so we can't really speculate on performance on a CPU/GPU basis, but its likely to be 20nm, so they'll net a solid performance benefit from just the process decrease irrespective of a uArch change, but I imagine it'll be a potent SOC.
In terms of Bay-Trail and performance/watt, it is just as competitive as the ARM variants; I said that before and I'll say it again, including the A7, and that is highly optimized for iOS.