yeah - if you do the motherboard standoff, you have to be in 3 slot mode, but that effectively puts the motherboard in the 4 slot mode position. The standoff mod takes up 1 slot. I double checked the clearance in the spec sheet, and I don't think your plan is going to work:
CPU cooler max height
3 slots GPU mode : max 74 mm height
4 slots GPU mode : max 54 mm height"
So your cooler could have a max height of 54 mm. Something like this Noctua cooler should work though.
My CPU waterblock barely fits to give you an idea of how little height you have available with the standoff mod.
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