I went from a P2 to an IA2 and I can (completely with unscientific bias) say that the FELT IA2 is better :)

With regards to the calpac. It's a little wonky and I feel it could be implemented better. That being said, I would rather have it than not. I personally use it to store backup gels and a multi-tool. During races I can fit an EFS liquid shot flask in there perfectly.