Regular or stiff flex on new AP1?

Depends how fast you swing your irons. Stiff driver does not automatically mean stiff irons, but it is the most common combination.

Here's my suggestion:  go use the Mizuno shaft optimizer club they've got.  It will suggest some iron shafts.  While it's designed for Mizuno clubs, I've heard of a lot of places using it for other brands.  It's not like you would need vastly different shafts in one brand from another.

Plus, the default shafts in the AP1s - the NS Pros - are lighter than True Temper Dynamic Gold shafts.  When I installed my current shafts, the suggestions from Mizuno included NS Pros in stiff or KBS Tour in stiff, but soft stepped.  It didn't suggest soft stepping the NS Pros.  So stiff in those might be a little whipper than regular in other brands.

Id go with regular.  That way you can only swing at 80% with more control shots and still get a decent trajectory.

I like the stiff shafts in my Ping i10's.  I'm not the fastest swinger out there (I get about ~150mph ball speed with my driver) but I like the firmer feel they have over my previous set which were Wilson Staff Di7's with the UST Proforce V2 iron shafts in uniflex.  It feels sometimes like the stiffer shaft lets me get away better with a hit that is slightly fat?  Try 'em both out and see which ones you like best.