The reason the K+N 57i gives such a performance boost is not because of the filter having incredible properties, it's because the standard setup in an F is really poor. For example, the majority of the gains between an F 1.8i and a TF 135 (+15ps) come from revisions to the air filter and induction system. TF 135 has a larger bore, simplified induction route with a trophy TB, VVC style inlet plenum and cams - this is where the gains come from - all breathing related.
You are basically repacing a really rubbish set up with a very good one (as opposed to a normal car where you would replace an adequate system with a really good one).
This has been proven by people putting aftermarket air filters on TFs and not noting any real gains - Sharkey can testify to this, he fitted an ITG Maxogen to his TF 160 and if anything felt it was slightly worse!
I can't emphasise this enough, the MGF in standard guise is being strangled by it's air filter set up. A K+N is the best mod you can do to the car from a value for money point of view ($ vs bhp).
The K+N 57i though is dependant on cold air from outside the engine bay (cold air is denser, therefore it contains more oxygen, more oxygen = more power). The K+N takes air from under the car, whereas the standard set up takes air from within the engine bay, there is a big temperature difference between the two intakes and this makes a real difference to the performance figures.
Basic principles are that the air entering the engine should be as dense (cold) as possible, it should not be restricted in any way (by the filter, or the bore of the intake pipework, or the number of turns in the pipework) and the air should be a laminar flow (as opposed to a turbulent one) and tuned to the engine intake cycle (the engine takes air in pulses as each cylinder goes through it's cylcle).
Therefore a filter fed by cold air pipes from outside, using an efficient filter medium, on a short, 'tuned' length of connection pipe will produce significant gains over a filter fed by luke warm air from inside the engine bay, using a reasonable filter medium sited in a long, narrow, torturous pipe arrangement with 9 ninety degree bends in it and a concertina section that makes all the air tubulent. QED
+8bhp on a 1.8i sounds quite conservative given the above. +15bhp is in my opinion slightly questionable, but i trust Rob to have been thorough on these tests (i have been present on all the subsequent ones) and the most important factor for me is that they were done by enthusuasts with no vested interests - the data is independant and this lends them a lot of credibility in my book.
Despite all the above, the K+N still isn't a perfect solution, but it is the best value. The filter cone is exposed and sits at the top of the (hot) engine bay. This means that although at peak power it is very good indeed, the motion of the car is feeding it with a lot of cold air. At other points in the rev range however, the supply of cold air is less (because the car isn't travelling as fast) and therefore the filter can suffer a bit from heat soak (it starts drawing hot air in from the engine bay to make up for a lack of cold air coming in from the induction pipes).
The above is the reason for the 'next gen' ranges of air filters, these are known as induction kits and take the principles of the K+N 57i a step further. These kits are a lot more expensive, but in theory will produce a few more bhp than the K+N and also do not suffer from heat soak, this is achieved through mounting the filter element inside an enclosure with a dedicated cold air feed to the outside. Because they are enclosed they cannot induct hot air from within the engine bay - they also benefit from simple, large bore induction pipework, tuned length intake tracts and efficient filter mediums. But the percentage gains over the K+N are not huge, just much better spread over the entire rev range, leading some to wonder if they are value for money. Kits include the Piper Viper, Mike Satur's Vader system, the Hurracane and the original ITG Maxogen. The Maxogen takes this idea a step further still, the Maxogen has a conical intake and this speeds the air up, increasing the volume of air into the filter, but at over 2 and half times the price of a K+N 57i, is it really worth it?
So, if you want the ultimate, get an induction kit, but be prepared to pay a lot for them. If you want a useful boost to performance and price is an issue, then the K+N 57i is an excellent way to go.
As a general guide, this is where the best gains can be had on an MGF from an air flow perspective:
Air intake position - poor
Intake pipework - narrow and full of turns
Filter - reasonable, some gains to be had here, but nothing major.
Connection to Throttle body - concertina, creates turbulence
Throttle body - reasonable (52mm Trophy TB is better)
Inlet plenum - reasonable (some gains can be had here by removing internal burrs). VVC manifold is capable of flowing more air (larger capacity), but it is questionable whether the engine needs this extra volume.
Inlet manifold - reasonable, important point here is to get it matched to the cylinder head, often there is a step between the manifold and the head
Cylinder head - poor, narrow ports and small valves designed for a 1.4 capapcity engine, significant gains can be had here.
Exhaust manifold - poor, primaries are too short and also have internal burrs impeding flow
Flexi-pipe - reasonable, secondaries are a better length, but could be better
Catalyst - surprisingly good, alble to flow more than the engine can expell
Rear silencer - good, some gains to be had here, but not much.
Priorities therefore are the air filter and induction set up, the cylinder head and the exhaust manifold - solve these and major gains are achievable.