Some in this thread have already made good points which I won't repeat. At the end of the day, the only way to truly compare the two headers on a level playing field would be same car, same dyno, same conditions.
From my own experience being in Canada (where the weaker Canadian dollar, shipping fees and taxes/duties are the enemy), the JDL 4-2-1 was much more cost effective for the gains I was after compared to getting an Ace A350 here.
Compared to the ACE A350 (non-coated) and a Delicious Tuning remote ECUTek tune, it cost me over $1000 less to get the JDL 4-2-1 (w/ full cerakoting) and a custom dyno tune with OnPoint Dyno.
Sasha said that it’s one of the highest HP NA BRZ’s he’s ever had. Car pulls strong right to redline and the torque dip is all but gone. These numbers are a 5th gear pull, with the stock intake, stock overpipe and stock front pipe still in place.
For reference, this dyno is a bit of a heartbreaker... stock BRZ’s typically make anywhere from 140 to 155hp on this dyno.
For more reference (and to contradict a bit on what I said above, but
reference is reference),
a 2014 FRS with a catless JDL UEL header made 166whp on the same dyno.
Do I think that the Ace A350 could have provided even bigger gains? A little bit more most likely, but I don't know for sure at this point, at least without seeing the results back to back on the same dyno. Also, for the extra cost, would it have been worth it? I personally don't think so based on my experience. However, YMMV.