I actually did this mod on the traditional felt with t guard true tongues in my Bauer matrix skates I converted to inline. The stock tongues had wear on the edge of the felt which gave me a hot spot on the outside of my shin around the cuff area on the boot. So I bought some true tongues, but in order for the middle of the t guard to line up with the flex point of the skate, I had to shorten the tongue which I did by cutting AND tapering the bottom of the tongue so it would feel more stock like. It worked great as I kept the Velcro system. I kept a flap of material from the stock tongues and added a hook velcro on that so I didn't have to sew into the toe box.
Looking back, I actually think the cat 9 or 9x4 tongue you have would've worked better in terms of overall volume. The traditional felt with t guard is quite thick but it still worked fine overall, and it's only for inlines that I wear like 5-6 times a year at most.
I also was able to look at the 3d tongues in store and they do seem very thick. And the fact that they don't look like they break in really at all, seems like of it doesn't feel right off the bat it's likely not going to get much better over time.