I have an 08 fit sport and at 63k had to replace all 4 coils, and plugs. That didn't completely fix it. Still stuttered at idle, so the dealer replaced the cat barely still under warranty at the time. That fixed it for a couple months until idle was rough again and my local mechanic adjusted the valves. All has been great, and I now have 130k. I thought I had a lemon. I don't think that"s the case anymore.

 

Adjust valves - replace ALL coil packs with known good new packs. I swear the gremlins travel from the bad packs to the good ones.

 

So, I have the same year and Model and the same problem and did all the same fix attempts with the same results. One simple change fixed all my ignition problems! All I did was tighten the gap on the spark plugs. I couldn’t believe such a simple thing would make such a drastic improvement. I had two error codes for misfiring at the time when I tried the spark plug gap change. All the previous problems disappeared. I went from hating this car to absolutely loving it! It’s been at least 3 years now and no ignition problems !

 

It's possible that a misfire caused a plugged cat. This is fairly common. Now that the ignition misfire part is fixed (hopefully!) your engine is starving for air since it can't push it through the exhaust.

My favorite way of checking to see if the cat is causing the lack of power is to take the upstream o2 sensor out. This gives somewhere for the exhaust gases to go. This is do at your own risk though because you don't want drive it longer than it takes to confirm power change (I recommend just one good acceleration run) Make sure that all wires/hoses/etc. are out of the way or protected from the hot exhaust around the new hole before you start the test. If you have more power with it out you obviously have a plugged cat.

Another safer way to check is the outlet to the cat once the vehicle is running will usually be a hundred degrees hotter than the inlet. If the cat is plugged than downstream will most of the time be cooler. Check with a cheap harbor freight temperature gun. This test is not always definitive that's why I like pulling the upstream.

One more way is to pull both o2 sensors and feel how much exhaust is coming out each one. If the downstream has significantly less than the cat is plugged. You can try putting the upstream back in and if the downstream doesn't increase significantly when you do this then its a good chance that it's plugged. Once again be careful do this on a cold car don't get burned and do it outside/well ventilated area.