cold air can hold more fuel, so a less restrictive intake with colder air will allow more air and fuel to reach the combustion chamber easily (in theory).
I have found marginal (at best) improvement from even the top end intakes until you reach "holy-crap-it's-gonna-blow" RPM on other vehicles. As for the XJ- I don't forsee anytime soon where I am gonna spin it high enough to justify a couple hundred bucks and I'd rather use the $$$ for a snorkle and be safe in the H2O.
.02 worth
--Shorty