If the engine was misfiring, you would have surely noted it on your 0-60 test run. I had misfiring occurring recently on my plugs that had 96k on them (2011- 3.5 N.A). No CEL initially either. The misfires were cuts in power delivery and caused the tranny to act funny and shift funny. Was very obvious something was wrong. Finally, after flooring it once (like a 0-60 run), I got a misfire code to trip. New plugs with silicone on the boots took care of it.
Assuming it accelerated clean and smoothly on your test run, I cant see misfires being the cause. I am not an engine expert by any means, but I would ensure your air filter is in good shape. Air flow is critical. I have heard of cats slowly plugging off, not allowing engine to breath with a slow loss of power as the cat (s) plug off more. I wouldn't go replace the cats necessarily, but it could be a cause, and at some point you would probably start getting faults. Maybe muffler is plugged up also. Or possible exhaust pipe is smashed, reducing air flow. Some easy things to check.
Also, did you remove intake manifold when you replaced plugs. Are you sure it went back together properly? Gasket did not come out of position, etc. An air leak here may cause funny things to happen. A quick google search shows loss of power and fuel economy a result of intake manifold gasket leak. I used a new gasket when I replaced my plugs. $10 piece of mind.
I swear my 2011 is sluggish also compared to when it had 40k miles (when we got it). But it runs good overall, with no CEL, so I'm not pushing my luck at this point.
