If you are going to do a straight hoist, dinghy remaining upright, your dinghy will want to sit on the swimgrid, no matter how wide it is. I use a pair of maple flooring boards (because I have a bunch) that i place like a ramp below the davits, one end firmly attached to a cleat on the davit support, the other end resting on the edge of the swimgrid. when lowering my RIB (750 # including 40 hp) it slides nicely down the ramps to the water, no more trying to stay on the swimgrid.
The depth of the swimgrid is about 30" or 32", and doesn't impair my ability to get the dinghy up or down.