Depends on things like:
-Gpu power draw. More power = more heat.
-The cooler's proximity to the side panel. The closer those fans are, the less air volume they're going to be able to push into the heatsink. Air is drawn by way of a frontal cone, not out from the sides.
-Chassis airflow. How air moves through the chassis with V or H gpu is different. Air can't go through psu shrouds either, so the air being ejected out the bottom ends up circulating back into the cooler.
Some heated air does make its way into the cooler in the horizontal position, but with vertical, plus with a psu shroud, the volume of heated air fed back into the cooler is greater.
TL;DR: I'd expect 80C+ under load.
Obviously, you're still welcome to try it. Perhaps it won't be as bad as I'm making it out to be...