Convergence theorem for Genetic Programming?
I am not sure that there is one Theorem to point to. Anyhow, you will probably be interested in the following reference, which summarizes many more recent results than those you mention in the question, e.g., for the (1+1)-EA with linear functions, quadratic functions, and monotone polynomials as objective functions (I know these are probably much simpler functions than you care about). Anyway the citations within should give you good pointers, via Google Scholar, to even newer results.

Stefan Droste, Thomas Jansen, Günter Rudolph, Hans-Paul Schwefel, Karsten Tinnefeld, and Ingo Wegener (2002): Theory of evolutionary algorithms and genetic programming.  In H.-P. Schwefel, I. Wegener, and K. Weinert (Eds.): Advances in Computational Intelligence Theory and Practice. Springer, Berlin, Germany, pages 107-144.