When talking about coils, the two primary factors affecting depth are size and type. As a general rule, the larger the coil, the more depth it can achieve, though there are practical limits. Type refers to concentric, elliptical, or any of the other varieties of creative windings out there. As another general rule, in ground where mineralization is not a factor, a concentric coil will achieve greater depth than an elliptical coil of the same size. Elliptical coils, on the other hand, perform well in highly mineralized soil and have the advantage there.
Your coils are all concentric (round) so the only real variable is size. The 800 coils (eight inch) will not give you the depth of the 950 coils (9.5 inch) but will give greater depth than the 5.3 (which is actually about 6.5 inches). However, smaller coils give better target separation because they are less likely to cover two or more targets at the same time than coils with a larger footprint. This means that smaller coils work better in trashy areas than larger coils. In very trashy areas a 3 inch coil would have a certain advantage over a 950 coil. Of course, another consideration is that no matter how trashy, if the targets of value are deep, a small coil is of no use since it can't reach the deeper targets. The most common solution is to dig up all the shallow targets, regardless of value then search again using the largest coil for the greatest depth.
While I like the 800 coils, there isn't enough performance difference to justify the expense of replacing your 950 coils. You would be better served getting a 3 inch coil IF you work very trashy areas. Otherwise, continue with what you have.
I used to have just about every coil made for every detector I had. When I went hunting, I carried at least two detectors and six to eight coils on lower rods with me. That really got to be a lot of stuff to carry around. Nowadays I carry one big coil and one little coil for each detector and that seems to work pretty well for me.
