I think FlexATX is too small and will remain so for the foreseeable future. This PSU claims 91% efficiency. Suppose you can make it to 95%. That means 1/20th of the PC's heat is dissipated in the power supply. For low noise, you want to minimize peak airspeed and have a moderate exhaust speed which is about the same everywhere.
Typical gaming desktop can get by reasonably well with a single 120mm exhaust. FlexATX has 40mm fan. 20 * (40mm)2 is about half of (120mm)2 . So you either have to run the PSU exhaust twice as fast as the main exhaust, or accept higher exhaust temperature from the PSU (to be fair, most people will, because they don't have temperature readings for the PSU to freak out about). And that's at 95%.
(Also the future of form factor is laptop.)