I was shooting in 1080 60fps mode, and approximately 45 minutes of footage is about 11GB. So, yes, you will eat up some memory in a hurry, especially if you use any of the higher-resolution modes.
I am including a link to some footage from my first dive with the mounted GoPro to give you an idea of how it worked out. It was on a ball mount on top of my camera housing connected to another ball mount threaded into a tripod mount on the GoPro to clear the sync cords and also allows me to lay it flat when I have the strobe arms folded and stowed. The FOV of the GoPro is less than the FOV of the 15mm FE I was shooting with on this particular dive. I just need to angle it down a few degrees, particularly when shooting things up close. The constant moving as I check the LCD screen, adjust strobe arms, and strobe power settings while shooting stills is pretty obvious, not to mention all the strobes going off...but I think it definitely can come in handy on some dives...