There REALLY is no better filter for an MFi RST than the standard Ford airbox with a K&N Panel filter in it.

The engine bay of any turbo car is VERY hot, but the RST engine bay is also quite tight, so suffers even more.

A heat-shield is pointless for a cone filter as the temperature of the whole engine bay is hot.

So, if noise is more important than performance, then fit your induction kit and also put a dump valve on.