I agree. If anything, it'll be due to a change in capacitance, not resistance.

The way that I check that everything is shielded and grounded correctly is to make sure there is zero resistance between all the grounded components and the ground wire to the output jack. So, if you've shielded properly, there should be zero resistance between the shielding and the common ground. So, shielding shouldn't add any resistance to the circuit if it's done correctly.

Capacitance and shielding is another matter (that I frankly don't understand yet). So I'll leave it there :\