Slight loss of power after replacing CAT
 

To start off, I have an '05 rx8 with about ~70K miles on it, original engine, no mods, most components are OEM. I've had this car for 5+ years so I know when something isn't right.

I had a check engine light on recently with a huge loss of power at all RPM's, and brought it into the shop. They said it was the CAT, so they replaced it with an OEM part. When I went to pick it up, they flooded the engine before parking it, and it took about 20 minutes and a power booster to start it. After driving it for the day it had most of the power back, but I noticed a slight loss of power at all rpm's, and a weird lag right where it's supposed to pick up at around 4500. It also sounds a little bit louder (like a growl) like it's getting too much air mixed in.

I looked in the forum, and decided to replace the spark plugs and wires. Didn't replace the coils because they only have about 10-15K miles on them. This helped a -little- bit, one of my spark plug posts broke when I took it off, and they all seemed pretty dirty so I thought that might be the problem.

But it wasn't... I have no clue what could be wrong after replacing all these things, I don't think my compression is going bad, I've taken care of this car and kept up on fluids/repairs pretty well. Oil up every other gas fill-up.

Could the people at the shop have messed something up ? Forgotten something? Or do I need to do something stupid simple like replace an air filter? I'm at a loss.

I dont really have an answer for you other than to mention that your coils could very well be toast even at 15K miles. Its pretty easy to test them though. Look on youtube. I couldn't believe how easy it was to test them.

There are a couple of warning signs here: it flooded. Healthy cars don't flood. They replaced the cat, but cats usually die due to ignition failure. Ignoring coils sounds unwise at this point. Did they show you the old cat?

Depending on what coils they are 15k is not crazy. The cheap stuff you can get on ebay is lucky to do 15 miles.

Also, have a compression test soon so you know what you're investing in.

Thanks for the reply. I went to a very basic shop, not the one I used to go to (I moved recently), so they didn't really tell me much other than what was on the bill. I kind of just assumed they flooded it because the dude didn't know how to drive stick shift that well (as I did when I first bought the car), but it could very well be bad quality coils, as I got them on Amazon without doing much research. I'm a half-assed car guy and I'll admit it lol.
I stored the car for the winter, so when I take it out, I'll bring it in to the shop and have them do a compression test, test and replace the coils, and maybe even look at the alternator. I've been getting some feedback through my speakers every once in a while, and I can hear something metallic grinding when the car is in idle, usually only when it's cold out.

I'd almost want to just double check the O2 sensor connections. Also look up what an OEM Rx8 cat looks like and make sure they didn't just slap on some off the shelf random highly restrictive 2in piped Cat they saved from some other car.

one thing to know is that Fouled Spark plugs can lead to the engine flooding
Bad coils can lead to poor ignition and higher chances of ruining plugs
poor ignition leads to unburnt fuel and oil being thrown into the Cat, leading to a ruined Cat.

Look up an LS coil kit, like the bennet built or BHR kit if you are planning on replacing coils.

Do the compression test first. Possible bad cat fried your engine. If thats the case you need a new engine or rebuild. You'll be extremely lucky if its just coils at this point...