IIRC, the feet are removable. In fact, I would advise replacing them for taller ones - with PSU exausting down and a fan mount and CPU cooler right next to it, this case should be raised high so it doesn't recycle hot air...
If you place it horizontally, like you're thinking of, it's not neccessary, ofcourse, but for normal orientation it might be benefitial.
@Cicciobello
If it works, if temperatures are good, then it's fine
But why this case for an ITX motherboard, why not Metis? Switching to ATX mobo soon?
I'm thinking of getting Thetis to replace my huge Define R4 until I upgrade the parts and switch to SFF. I already got the SFX PSU and I'm thinking of hooking it on the HDD tray above and cutting the fan hole below, where the PSU is normally placed, to have a straight airflow for the CPU. I don't know if my 240mm long GPU will fit next to SF600, though...
Perhaps is better not to touch anything, my current parts are not hot anyway (i5-4670K with bigass Phanteks PH-TC14PE cooler and GTX 1060)...