Maybe its in my head, or maybe I'm just looking for reassurance because I did't notice it open up as much power as I've read.

I installed my 034 high flow cat this past weekend with a vibrant performance O2 spacer. Going into it, I didn't expect a whole lot. I assumed I'd feel like the motor was a little more open and responsive. If anything, I feel as though I've lost low end boost and its changed the boost to peak higher in the RPMs. The throttle just doesn't feel as responsive as it was, it lags a little bit before building boost, not as torquey, but I do notice more power up top. I did notice my MPG reading in the car is complete crap now, assuming this is from the o2 spacer, although it did save me from a throwing a code. To be honest, I felt more gain from my CTS FMIC than this HFC. My girlfriend says she doesn't notice any difference in the car, although she also doesn't drive it as spirited as I do.

Should I be checking anything or does this sound accurate with the boost being slightly delayed after installing the HFC?
Should I be concerned about why my MPG reading went from averaging 25MPG to 15MPG or is it just the o2 sensor spacer?

Just looking for sound advice, thanks all