I almost say "This has been beaten to death her". But I say this: It is a well known fact that Porsche choked the Boxster engine on purpose. Just do a search on that. A high flow air filter will help to open things up but will it do what it is supposed to do as well as the one it replaces? To filter the air and stop all small particles from entering the engine? Maybe if you use the type that you oil but then you get a lot of issues with the oil getting on the MAF and messing up every thing. So I will not have one of those on my car/engine as I am concerned that it gets proper clean air. The 987 air box has proper filter with much larger surface area than the original 986 filter and that is only one of the benefits. The piping is also more open, the box has bigger volume (and bigger MAF house as mentioned). And it is a proper "cold air intake" as opposed to some aftermarket intakes that claim to be, but in fact get some of the air from the hot engine bay.
One thing that I failed to mention is that you should also do the 996 throttle body upgrade and 997 plenum at the same time as you do the 987 airbox and do something to open the exhaust. You don't want any weak links or bottle necks (as we say in my native language) in your stream of air through the engine. I think all these things work together and you only get full benefit from each, when you do them all. What you do to your exhaust depends on the state you live in, I am firm believer in performance headers, but they are down right criminal in some states, as known. And don't forget that exhaust can be tricky you can rob the engine of power with some expensive aftermarket systems.
If you are not aware of this tread
her about 997 plenum and 996 throttle body, I suggest you spend half a day of your life reading through it. You will enjoy it, it is fundamental and real interesting reading.
And to answer your next question, my gussimate based on my 0-60 tests with an g-meter is that all these mods togeter give the S-engine about 30-35 extra horses. Worth it? I don't know. But it is dam fun process doing them all.