By putting the number of arguments onto the stack, we end up making the stack larger for every single function call. We also require every access to any stack variable to do math to figure out where on the stack it's arguments are.

That seems an insane overhead for the common case... If this really does make benchmarks faster overall, it suggests an even better solution might exist out there that does not add overhead for the common case...

Perhaps it might be worth keeping track of how many arguments a function is typically called with, and recompiling code for each case? Then all the checks for argument counts can be entirely removed in the optimized code.