I'll take a stab at an answer.
First, I do not have a tune. I'm on stock ECU.
Second, I do have a catless dp/rp exhaust system that does raise boost about 2 pounds. So I can usually run 18 psi with no creep in the higher gears, and especially in cooler weather do run into load cut with transient spikes in the 20-21 psi range that immediately taper back to 17-18 and hold to my preferred shift point of 5,500-5,800 rpm.
With my intake and exhaust mods I consider my car border line for needing one step colder plugs. Most experienced people say that once you go up about 40 whp in power over your stock heat range plugs or increase boost by approximately 2 psi, you are a candidate for colder plugs. I'm at that point.
Third, Cobb's maps for AP are designed and tested to work best with the one step colder plugs. It would make sense to follow their recommendation. You will get more predictable results and will be better able to address any tuning issues without wondering if the plugs are creating issues of their own. One less variable to contend with.
The thinking is that you want to run the hottest plug you can without getting into knock retard with the engine pulling back timing to keep you from getting detonation. Detonation can kill your engine.
The thinking is also that you want to run the coldest plug you can without fouling the tip. If you are not getting a hot enough spark and not completely burning the mixture, you get random misfires and/or foul the tip of your plugs. The colder plug will help prevent harmful detonation and help to keep your the timing advance your tune wants to allow, provided you do not pull so much heat away that it fouls on you. When your ECU pulls timing to try to avoid knock, you loose power.
So, you want to find that happy medium in which your plugs don't foul, but you are getting extra protection against detonation/knock without pulling timing back. These are not mutually exclusive concepts.
For me, running in a Deep South high temperature environment which can exacerbate the problem, and since I'm right on the edge of needing the colder plugs, even without a tune, I decided to go one step colder. This works for me.
I'd rather run a risk of a misfire or fouled plugs than run the disasterous and expensive risk of detonation or losing even more power by having the ECU pull timing back.
When in doubt, I'd rather run the risk of fouling plugs and having to clean them, than run the risk of burning a hole in one or more pistons.
For you, since you already have the dp, aren't you seeing a couple pounds more boost than stock already in combo with the increased breathing on the intake side with your CAI? You should be. Taking that into consideration, I'd say that you should consider getting the colder plugs and put them at the same time that you activate the AP and start playing with its maps and tuning. Cobb wants you to do that. Cobb will probably tell you to narrow the gap down to .028 or even .026. This will tend to help keep the spark intensity up in the high boost, rich fuel DISI environment in which they will be asked to fire.
Others may disagree. Maybe a word from AP users? I would doubt you would find very many of them still running stock heat range plugs, but I defer to them.
Tracking shows that they will be in on Monday so hopefully I will have some time to install them that evening.
