Let's be careful about generalizing here. An 8/31/2 AP is most definately not going to be stacking power over aim with the access to the 9% aim skill and lack of 6% tech crit a 4/6/31 Pyro has access to.
Why would anyone run AP? Even in PVE, AP is just sub-par when compared to the amount of actual DPS that Pyro can provide. To say, "I'm AP and my damage is always on the top of the chart," is to say that no one else on your team is doing very good. AP just cannot pull the damage that Pyro can - period.
As far as the amount of expertise, take whatever you can get away with having for greater damage dealing potential. But this idea that there is a sharp DR at 1200 is bit overexaggerated in my opinion. I put the links below to some of the graphs that plot out expertise's bonus conversion for damage done, received, and healing boost respectively. Make of it what you will, but I wouldn't refer to it as "sharp."
Using Advanced Reflex Armoring 27's in the Belt and Wrist along with Hawkeye crystals in the MH and OH provide a larger damage increase than stacking Expertise over 1200. This is simple math that can be done with using a calculator. Doing so, you only lose about 1.75% damage reduction, from ~1390 Expertise to ~1200 Expertise.
Slow down here, we cannot make this a blanket conclusion about all PT dps specs. Aim adds approximately 0.2 damage per point while power adds 0.23 damage per point. Aim additionally adds a small amount of +crit that is not on the crit rating DR curve.
Actually, the Crit Rating gained from stacking Aim does have its own DR table. That's been a known fact since near the start of the game, when people were gearing up in January. Also, pushing high Crit Rating does not make you deal more damage - It only gives you the potential to roll a critical hit. That's why DPS (even of other classes) goes Power heavy, along with Surge. Even for PVE, your crit rating should never exceed 300.
Now the gentleman to whom you are addressing uses a 4/6/31 pyro build for PVP which does not have access to the 9% aim skill. With the large amount of +crit chance skills and the 4pc eliminator set bonus, it is reasonable to choose the power favoring option for more bonus damage. Additionally, there is no such mod that has both high aim and high power.
Steely Resolve should only be used as a bolster for PVE damage mechanics. You should still always stack secondary stats heavier than primary stats, as any DPS. Having the 9% increase to total Aim only helps give that little extra, since it is used in place of TD builds (which burst is not necessary for pve). Like I said, no DPS PT should ever be stacking Aim.