i believe (someone correct me if i'm wrong!) the heat shields near catalytic converters are there to retain heat and get the temperature up.
The hotter the exhaust gases the better. Never really thought of the heat shields keeping heat in the cat., but your point seems valid. That's why companies ceramic coat exhaust components, at least the good ones. The ceramic insulates the pipe, trapping the exhaust gases in, which keeps them hot and doesn't allow dissipation of heat. Hot gases flow better than cold and we want the exhaust gases out as quickly as we can. However, this is arguable, but I think 2-3 psi backpressure is optimal on N/A vehicle. My friend just re-packed the Vance & Hines on his sport bike and said he has noticeable improvement on the low end.