I think you are talking about the 240AIO + 3-slot GPU configuration.
My opinion is, the thermal performance may not be as well as what you think it should be. And custom loop has the extreme flexibility and possibility to give you the better performance.
Maybe 3 more fans on GPU yields better thermal performance. But you need to know that all heat is dissipated into the case, which needs to be exhausted by the top fans. That means your CPU will not be cooled sufficiently.
In addition, custom loop takes the advantage that it's deadly silent. You only get two fans and a pump, which is much more silent than a 240AIO + 3-slot GPU.
Also, custom loop is much stronger than you think. At least it's better than an AIO with the same radiator size. With some tweaking (Undervolting and maybe decreasing the frequency a little), a 240 custom loop can really do most things.
About it's possibility, I'm also planning to build a custom loop and I've already got a feasible plan to have much larger size of radiator in T1 (Maybe 240 + 280 or at least dual 240). With that you can overclock both a 3950x and a 3090 to the almost highest point. That's what only custom loop can provide in a case small like this.
