I agree with everyone else here.. at 13psi and 300whp, you shouldnt even be close to maxing out those inkectors. avoid that tuner and go to someone else, it sounds like he doesnt really know what he's doing. Its not worth a motor trusting this guy

 

when we were tuning my buddies car, with 1300's and 550's we set the injector sizes in the pfc settings, but there was ANOTHER setting that you need to change also that we missed. we had the same problems as you are. i'm suprised no one else has caught this. i'm not saying that this is your problem, but we were running into the same problem until we switched the setting.
i wish i could tell you what it was, or where the setting is, but i dont have my pfc anymore, and my datalogit and laptop is in my other car at the shop, and everybody is gone to SEMA. its been about a year since this happened, so i'm sorry my memory isn't good enough. check the board for something like this, i think this is where we found it. on a side note, we made 354rwhp on seq. twins at 14psi on a mustang dyno after fixing it. so you should have plenty of room.