Catch already does assertion checking (1). Its runtime on moderate to small programs (HsColour in particular) is far less than the time GHC takes to compile them, and I still have no idea what its runtime is on enormous programs (2). An analysis can be whole program and can be slow, one does not imply the other.
But the primary problem with Catch is that its analysis not well defined. I have no guarantee regarding the existence or not of false positives or false negatives, as Catch has no underlying formal logic to guide such reasoning.
If Catch says your program will not crash, then it will not crash. I even gave an argument for correctness in the final appendix of my thesis http://community.haskell.org/~ndm/thesis/ (pages 175-207). Of course, there are engineering concerns (perhaps your Haskell compiler will mis-translate the program to Core, perhaps the libraries will be wrong, perhaps a bit in RAM will flip due to electrical interference), but Catch has a formal basis. I guarantee that there are safe programs Catch can't prove safe, for a proof see Turing 1937 :-) Thanks Neil