I would say the 5x8 coil ID's some targets better (silver dime tid on my machine is 84-86, where clad dimes read 82, and deep too). Depth is almost as good as the stock coil, and I would say that your swing speed can be fast for shallow targets. Just to give you an idea how sensitive the little coil is, I can be walking with my detector in one hand, and my shovel and propointer in the other hand. If I lift my coil about 8" off the ground, it can detect the shovel and propointer I'm holding in my hand! I would say this coil doesn't cover lots of ground, but it hits deep and would be suited to finding caches up to 3' deep, depending on the size.
It's way easier to see all the stuff in the ground too, because of the faster response speed. The 5x8 is my go-to coil now, but I also have the sniper and the 9x12 and the stock coil. I use them all, in different situations, and they all work well, but the 5x8 RARELY comes off of my machine now. It's already paid for itself in a few days (just haven't uploaded all the finds yet!).