Not being an expert, but rather thinking logically, then I presume it would depend on (i) the volume of intake air the engine requires, (ii) the volume of air the extra intake supplies and (iii) the volume of air the cold air is being delivered into (i.e. dilution factor).
Difficult question to answer off hand, without doing alot of measuring! But bottom line would have to be anything is better than nothing, so if like alot of people, you\'ve got big foam filters sat un-heat shielded and potentially drawing hot under-bonnet air, then yes, it may make some difference! I guess the ideal would have to be some kind of heat shielded box, and then good ducting so that the majority of the intake air is cool, i.e. the hot:cold air ratio favours the cold rather than the hot!