Car feels very low on power. At 40mph it will very slowly speed up after a long shift. Same thing happens @ 80mph, but rpm's stay at 4k RPM's and very very slowly increase.

With tapshift the car never really goes into full WOT where it will change gears very quickly. I'm sure if I was racing my car @ 40mph (without tap shift) I would lose to a GT. And I'm also sure that I would lose to a GT if I had tapshift activated because of the loss of power.

Is this a tranny issue or a vacuum leak? (Or am i completely wrong on all of these?

Recent maintenance:

Changed plugs to Autolite 104's inspite of this issue to try to fix it.

Had dealer put in new downpipe section + new cat.

I also tried cleaning MAF sensor because of this issue along with changing oil to 10w-30 royal purple, no help. (Normally ran Mobil 1 10w-30)

What do you guys think the issue could be?

Sounds like a clogged cat

Funny you mention that, I just had a new cat put in because I thought that was the issue, it is not.

Definatley not a vacuum leak.

Hows it idle? Might want to take a peek to see if theres any hidden DTC's.

Tap shift = worthless.

Sure, maybe worthless in gaining any performance out of your car, but what if you just want tap shift to work properly? DTC's?

Diagnostic Trouble Codes.

Go to VatoZone and scan for free, using the OBD-II port.