Power loss/Knock/Rough Idle - Any help greatly appreciated!
 

Symptoms:
- Rough idle
- Lots of power loss after 5k RPM, most noticeable on the highway
- After 5,500 RPM, a slight rhythmic knocking sound that increases in volume as RPMs climb

Mods:
- AP dual resonated mid-pipe
- Greddy SE catback

Additional Info:
- 2005 MT
- 96k Miles
- Previous owner had the engine replaced at 60k via warranty
- I had mazda do a full inspection of the car and they said the only issue at the time was the motor mounts were wearing out. That was about 6 months ago.

Ok, I'm really getting worried because I feel like my soul or something is tied to my car and with it sick, I honestly feel like crap. I don't have the OEM cat and when this started about a week ago, my initial plan was to pick up a high-flow cat before taking it in. Well, then I realized that I'd be looking at $700 for the SP high-flow cat and haven't found an alternative high-flow out there. At this point, I'm not sure if I should just take it in as is or go ahead and pick up the SP high-flow cat beforehand. My main concern is the engine is going out or something drastic and at 96k miles I don't want to take any chances of voiding the 100k warranty.

Anyhow, I've spent the better part of today searching and searching and while I've found isolated situations where people will have power loss, rough idle, or knocking at high RPMs I haven't been able to find any posts that have that have all of the symptoms my car is.

Any advice would be most welcome. Thanks a ton!

-Mike