While I agree that HS-6 is a very good powder for full-power (or very close to it) 9mm loads, I certainly do not agree that it is "fluffy".
HS-6 is a ball powder. As are most ball powders, with the possible exception of flattened ball, it is a dense powder.
It is also much, much slower than Sport Pistol. I've never used SP, but from what I can find out it's a fast powder, along the lines of A#2, or even TiteGroup.
HS-6 is much slower. In betwixt A#5 and A#7, slower than Unique or Universal.
A double charge is evident because as a slow powder one needs more of it to attain velocity.
The softer recoil push is from the slower burn of the powder.
Just exactly how long does it take to get use to it ? I’m on my first day and about to quit
It’s big a overnight feat to transition to AIWB carry.Springfield xd subcompact we the people kydex holster
IMO, your holster choice is going to be an issue. The WTP is not built correctly for effective AIWB carry. It will never be optimal. If you give up on it and decide to try another, look at Keeper Concealment, JM Custom Kydex, Dark Star offerings.
For most people a longer model works considerably better than a compact or subcompact.
I’ve carried full-size firearms exclusively AIWB for some time under a polo shirt. A Wilson EDC X9 in a X9L holster. 4.25” and 5” M&Ps in a 5” holster, and currently a G34 length holster with a 17 or 45 with a dot. I carry them in this position all of my awake hours daily. I drove 6000 miles in 30 days recently carrying in this position. Being able to adjust the holster to your needs is crucial. .060” can make a world of difference. You have to find that sweet spot.
Belt is also a big deal. I highly recommend a Foundation belt from EDC. It’s made locally (but well known across the country) and I made the switch to it about a year ago. A wilderness instructor or frequent flyer is a close second.