I'm not sure trimming the shield is strictly necessary to get the benefits of these. Porsche runs a splash shield on some of their (admittedly bigger) 911 front setups, like this 997 GTS:
991S:
As long as you're getting more air somewhere near the annulus of the rotor where the rotor is pulling air so it can suck it out through the internal cooling vanes, you're doing better than nothing
Will try, test, and report back when I get a chance