To be honest, from a hardware perspective, the dual core A5 chip in the 4S and the iPad 2 is hardly utilized even until now. The A5 chip still has massive graphical power with its dual core GPU.
Yes, the A6 chips does things way faster than the A5 chip in some cases. But hardly nothing really slows down the A5 chip.
Thats what happened to the Galaxy S3 actually. There is really no point in including a Quad core CPU when your GPU can't keep up. Yes, there will be performance improvements, but in modern times applications rely more on graphical horsepower than computation and calculation.
That means, although the quad core chip in the S3 beat the iPhone 5 in general benchmarks (barely) , it loses out terribly on the GPU side. Thats where iOS and the iPhone/iPad shines in - Graphical horsepower.
With new alterations of OSes focusing more on improving the UI and everything, GPU instead of the CPU comes into importance.
And hence I can confidently tell you that iOS 7 will hardly slow down your iPad 2 by much. Maybe a tiny little, but unnoticeable. In fact, with software improvements and tweaks, Apple can it make it run faster on iOS 7.