[spoiler="EDIT: DO NOT FOLLOW"]
(the blue line is just to highlight the PCIe area of an mATX board, not a giganto unicorn 3KW PSU)
[/spoiler]
i don't think even a 1 slot card would fit with regular ATX PSUs with the current bottom floor definitions, but I wouldn't reckon it'll be a big issue to give a few more millimeters to accommodate. ditto true 2-slot cards for SFX. note where the outline of the 2-slot card end relative to the next PCIe socket
that being said, not many mainstream cards are 2 slots wide, so you might have a point there. it's the same issue that mATX boards themselves would face, since mATX boards capable of two GPUs are usually of 1-slot spacing (for 2-slot cards), even when you treat them like ITX boards (ignoring mATX boards with x16 at slot #2, which makes the situation worse) or straight up use 3-slot cards on ITX boards
there is some trade-off with the positioning of the PSUs.
- if it's above in the front, it'll be the NCase M1 or the Cerberus.
- if it's at the bottom-rear like the CX2 it'll limit GPU choice due to slot-width
- if it's at the bottom-front, it'll limit GPU choice due to card length
heck, if you see the image, a 3-slot card is all the case will do with an ATX power supply at the bottom-rear (although there might be issues using true 3-slot cards with motherboards with the first x16 socket at slot #2, which is ALL (but one) of the TRX40 boards)
