No offense but all these questions have been answered numerous times in the threads you have already read. A few post down there's a newer intake post about afe or kandn, giving the same answers that have been given 100 million other times on just about every car forum known to man, and will probably continue. All the intakes are simply filters on a stick and will all produce more engine/turbo sound. They're all going to gain minimal performance, and personally I only recommend them if you're trying to maximize power from your bolt-ons and it's your last modification (ie you have already upgraded the intercooler, downpipe, flash, etc). Get the one that you like the looks of the best or can get the best price on. As far as a drop-in filter, the hundreds of a second you mention are negligible and can be attributed to many other factors. Furthermore those are only results you'll see from a new k&n filter. Time and time again there have been many many tests that have shown that as soon as the k&n filters get the slightest bit dirty they actually flow less than a stock filter. Save your money for something else.
Edit: I should say minimum performance gains on a four-cylinder engine like this. There are some cases with larger V8 engines/ higher performance engines, that sometimes have restrictive intakes where you can see some actual gains from intakes. These instances is a rare as many factors as of late have optimized and increased efficiency in engines as much as possible to conform with current laws and standards. Case in point here's a $1400 intake for new Corvette power gains? 14hp 13 left.
Edit: I should say minimum performance gains on a four-cylinder engine like this. There are some cases with larger V8 engines/ higher performance engines, that sometimes have restrictive intakes where you can see some actual gains from intakes. These instances is a rare as many factors as of late have optimized and increased efficiency in engines as much as possible to conform with current laws and standards. Case in point here's a $1400 intake for new Corvette power gains? 14hp 13 left.