I guess to better phrase my question is screen tearing likely at high frame rates? I know at low frame rates you're more likely to get it as the monitor will likely be at a higher refresh rate than the card is putting out.
Tearing is more common at higher framerates. Tearing happens when a new frame is produced while the monitor is in the middle of a refresh cycle. When the framerate is higher, new frames are being produced more often, and there are more opportunities for tearing to occur.
G-Sync and FreeSync don't operate above the refresh rate of the monitor, they only operate while below, so are beneficial at low framerates (where tearing is less common anyway). Tearing prevention is not the only purpose of G-Sync, and in fact it's not even the primary purpose, it's a secondary benefit. The main purpose of G-Sync is to improve smoothness at low framerates.