Heat proof reflective matting on diffuser?

[ShieldsOnTour]

OK, as part of my car "rebuild" I'm going for the Subaru diffuser mod. Am worried that the proximity of the plastic diffuser to the cat it will need some kind of heat proofing.

Found this:

http://www.ebay.co.uk/itm/UNDERBONNET-TURBO-EXHAUST-HEAT-PROOF-MAT-HEAT-SHIELD-BARRIER-CLOTH-PT2-/151202350042?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item23345cc3da

Will the deflected heat back into the engine bay degrade performance at all. Or does the temp of the engine bay make any difference as the air is being pulled in from outside?

Thanks,

Tim