For 300W consumption, you'd have to have a GPU + 71mm cooler to dissipate that much heat. That or a full sized cooler. At anything around 3L, it doesn't make sense to support that much power since you wouldn't be able to cool it. For CPU, you'd need either a 140 rad or a full 160mm tower to cool 300W.
Mini-DTX motherboards don't actually exist, as the purpose is to have 2 slots instead of 1. I have not seen any consumer boards out there. Flex-ATX is quite easily available. They're labeled "mATX" on the market as I've linked.
If the Pure grows by 10% volume you gain the following:
1. FLEX ATX support (1 x16 + 2 x1 slots)
2. Low profile GPU + UHP-200-12 support
3. 4x SSD + UHP-200-12 support
You lose the following:
1. Increase of volume by 10%
2. 7mm loss on CPU cooler height.
RPS-200 support still remains.
Of course, additionally it'll be made in USA. There may be a small price increase but hopefully a good design can prevent that. We'll have to see.
