On this case, I will respectfully disagree. I've done BOTH K&N upgrades and PAnel filters on vehicles and the CAI's actually help more with MPG and throttle response in all of the vehicles I've had.CAIs do nothing. Just upgrade the paper filter to a K&N filter and you will get the same performance increase, In most cases more because CAI kits come with cheap filters.
Apparently, you haven't seen my thread:All elantras have to be made out of an accord or a custom kit. And everyone I seen on an elantra ends up behind the battery, where the battery used to be (there is no cold air there, actually thats hotter air than the stock system) or in the fenderwell.
The stock air system gets air semi-directly from the grill, and if you wanted to you can route that flex pipe to the grill and get pure cold air. You can't get much colder than than directly from the grill.
Whether you live on a paved road or not sticking a filter out in the open is going to make it get dirtier faster. Which will cause you to lose performance at a faster rate than the stock system. The so called "restrictive" stock system will then in turn have better air flow.