there should be plenty of cold air to your airbox without the need for a deflector
depends on the country and climate.
i know my 827 was suffering in the heat in italy - not so much its cooling problem - but generally down on power, and i know several teams (one of which actually shows you on the DVD) were feeding more cold air to the engine during the final day of the rally using various things.
also, when we had that really hot spell in 2003, the mechanics of 1906 had just been finished and she did not like the hot weather in the slightest. she was definitely down on power.