I was under the impression 2.75 slot compatibility was guaranteed because future GPUs are getting taller, not longer. If DAN is going for a hard riser for future compatibility, then 2.75 slot is a must too.
Edit: One small niggle I take with the design being intended for the rad to be at the bottom is that in an AIO, this means that it is likely air bubbles will work their way into the pump, especially over time should any evaporation occur.
This will either result in a lot of pump noise, or will even prevent liquid from being able to flow. That is why the recommendation is to always keep the rad above the pump, so that gravity will stop the bubble from being able to get out of the rad.
I know you can flip the case to make this a nonissue, but then the PSU trunking you've been designing isn't going to work as well, no?
Just something to think about. It doesn't seem like a good idea to have it as the default.