That looks like one of the best ways to accommodate an X300U for AIWB that I've seen. I just couldn't get used to the length for long periods of sitting and switched to the TLR7-A with a Tenicor Sagax Lux and I'm very happy with it.
The only issue I've had is that the built in wedge is more slippery than a foam wedge so it would eventually start to move or allow my undershirt to move past it and untuck itself. I stuck some soft side velcro on the wedge to give it more texture and that solved the problem.