well i don't know for sure, but i'll relay what my mechanic said when I asked him the same thing about my M800.
Performance wise, the best bang for the buck is sprocket adjustment. In second place, an aftermarket exhaust, K&N air filter, and dynojet. So if you've done the sprocket thing, you should save for some nice slip ons like sil motor (i've got carbon termi's myself) and THEN dyno w/ a K&N air filter.
He said dyno-ing and getting the air filter without an aftermarket exhaust provides some benefit, but he strongly recommends waiting for an exhaust system.
So in short, you'll get a small boost w/ the stock cans, but a very respectable boost w/ aftermarket.
Again, this is just what he told me. He could be wrong, but I thought you'd like someone's opinion.