I've got a 2015 F150 I've been installing some bolt-ons to and enjoying for the most part. (aFe "CAI", CAT back exhaust, RXP catch can, once I get this solved im putting in a Mishimoto intercooler & hot/cold side pipes)

Recently though I've been having some issues with the truck.

After I installed a catch can (and finally fixed the check engine light) I've been having some delayed response issues. These are more pronounced in the overdrive gears (5 & 6), and extremely so when carrying a load.
Basically when I go up an entrance ramp (or any included slope) and give excessive gas it starts to lag a bit and I can literally feel the miss fires lol. You can also feel the entire power train rumble as its sending the power to the wheels towards the end of the lag. If I keep pressing it will usually force its way up and get moving, but it takes a few seconds.
Additionally during acceleration when the turbo spools up it doesn't want to fully spoil up.
Before I could get ~12-14 PSI at "max" boost, now im lucky if I can get 8 PSI.
Normally she gets up to ~6 PSI and it fluctuates a little at the "top" of each boost cycle.
(I had a similar problem (not the turbo issue though) but much worse on my old truck and the fix after literally 3 years of mechanics trying to fix her was just to change the coil pack and it fixed everything...)

Recently after I've changing all 6 spark plugs (checked spacing @ .030) & replacing all the coils for giggles and after taking her out tonight and really stretching her legs she feels much more responsive and I even managed to make her boost close to 12 PSI for the first time in a month or so.
I can still feel it during some of the higher gear runs though. Its a lot harder to notice though.

I took it to a mechanic and after running some code and he recommended plugs & PCV sensor replacements plus the #4 coil specifically. which i did and its better now, but still not where I want it to be.
He also said it might be the "CAI" because the sensors aren't programmed for the increased air flow and that's the root of the problem. I personally don't agree because I didn't have any problems after i installed the "CAI". Also I've been told these modern engine computers "compensate" for such things thus why they say you don't need a tune after installing most of these aftermarket parts.

Anyone got any ideas what it could be?

 

After I installed a catch can (and finally fixed the check engine light) I've been having some delayed response issues.

Go back to your CEL fix, and then pull the catch can and reinstall the OEM set up. See what happens.

Speaking of CEL, any codes?

 

Original code was crank case air. After I switched around the sides of the can the dash light went out, but when I pulled codes myself it still showed the same and a misfire in cylinder #4.
currently it’s showing nothing.

Im driving up to Huntsville tomorrow for work so it should make for a good test.

 

Well, to start with, the aftermarket CAI units don't flow any more CFM into the engine than the oem setup..... unless you increase the stock engine bore, stroke, rpm, etc. and the oem cai will typically flow at least 10% more cfm than the engine can ingest. It's true that the oem ecm software can adjust for up to 25 a/f mix, what it can't adjust for is the poor engineering such as repositioning the intake sensors thereby giving false readings or fitment issues that induce leaks (air related) during high flow demands. IMHO, put the oem cai back on and see what happens.