No question gun fit is important. However, as suggested above, some people adapt to significantly different dimensions better than others. Ted Lundrigan, a Minnesota grouse hunter, discusses his grouse guns in a book called "Hunting the Sun". He talks about using a Westley Richards 20ga, a Remington Model 31 Lightweight, and a Parker 12ga with a dogleg stock. I'd submit that anyone who can switch back and forth between those 3 guns when hunting grouse--where, to borrow a phrase from OJ's lawyer, "if the gun don't fit, you will not hit!"--is about as adaptable as one can be.
Cast is one dimension which, I think, is less reliable than LOP and drop when it comes to the measurements you need in a gun. I've found that I can shoot guns with no cast and guns with quite a bit of cast equally well (or poorly). Yet on other guns with a fair amount of cast, I know immediately when I mount them that they won't work because I'm looking down the right barrel rather than the rib. Seems to me the reason that cast isn't necessarily as fixed a measurement as either LOP or drop is because comb thickness plays a significant role. It can function either to negate a lot of cast or to emphasize whatever bend is present.
