I have a weihrauch on my FX wildcat mk2. The gun has a 500mm barrel, and a 300cc tube, which extend under the moderator. The weihrauch has just enough clearence to the tube, as its OD is a litle smaller than most other mods. It works very well, and is well worth the money if you like the lenght of it.
My favorite "expensive" mod is the huma modular mod. What I like about it is that you can add, or remove sections, finding just the right balance of keeping the gun short, and at the same time silent enough. With a traditional mod the gun might get longer than nescesarry, with a longer mod than needed. Or it gets to loud, because the mod is just to short. So with a traditional mod, you are stuck with what you got, as you can not change it.