I used vertical mount gpu's two times. For that you need a pci-e riser. The performance is quite the same (i see 1-2fps lower in benchmarks) and a bit higher temps 2-3C more. With rtx 2080 i had problem with the pcie riser i was getting black screens , freezes and weird beeping sounds so i ended installing it again directly to the motherboard. With my second pc with 1070ti that i still use the gpu vertically i did not face any problem at all.