Of course, to "really" solve this problem you'd have to recognize "mutually unused" arguments: foo(a,b) calls foo(b,a) and doesn't otherwise use the arguments. But foo(x,a) which uses x and calls foo(a,x) uses both. Not difficult to solve, and a few seconds' thought puts it in O(m*n) with m recursive calls and n parameters... but probably not worth the trouble.
Sorry, how is this O(m*n)?
Hmmm... represent the parameters as nodes in a directed graph. For each of m recursive call sites, process each of n parameters and add edges from formal parameter name to passed argument (i.e., if we have signature foo(a,b,c) and recursive call foo(c,b,c), add edge a->c; self-edges are unimportant). This step is O(mn). Then, assuming we already know what parameters are used outside of function calls, flood-fill the graph from those nodes; this is O(n). O(mn+n)=O(m*n).