I just added "Royal Purple Octane Boost" in my '15 XT to my 93 "BP" fuel and the surging has stopped after 20 miles of driving.I've seen this thread for a long time but have now only started experiencing the issue at 120.5k miles.
Lately I've been driving in the middle of Texas in 95+ degree heat, 300 mile trips, during the day at 25-30% throttle and 8psi boost - started feeling the surging.
After I began to notice it it's now happening during evening drives as well.
I never had the ecu update for the surging [Bulletin #:11-164-16] as @Giantdefy mentioned. I'll for sure do the MAF and MAP sensor clean and if that doesn't work I'll probably take her in and asks how much the update will cost (any one know cost? since it's not a recall)
I did the new clamps WAY early on in 2014 at around 10k miles. I'll check those too.