AFAIK the filter box/ filter on all engine sizes are the same, so if you have a smaller engine (1.9JTDM or a 1.8) then you probably will see little/no advantage to fitting a better air filter (as they are designed to cope with the air flow for more demanding engines), once you start mapping, increasing boost etc, then they will be more important.
Overall you are correct, better flow should allow better performance, though I think cars now have much better designed airboxes than in the past, so it seems to bring less advantage (I used to have a mk1 punto and a K&N air filter helped smooth out the power noticeably, doubt it had much impact on power mind). Some people claim that the aftermarket oil impregnated filters let in too much dirt, but over time I've not seen any evidence of anyone suffering engine damage or issues due to this.
Size will have an impact, but the material will probably be more important (as they are all within a couple of mm size of each other), also remember they may have advantages at different points, one may allow more flow at maximum demand, whilst another may allow better flow at lower pressure (across the filter), meaning it could give advantages at low rpm (hence my punto having a smoother power curve).
This graph helps explains this (increasing pressure along the bottom), most of the lines show more flow as pressure increase, a couple of times the lines cross (showing advantages at different differentials).
As for why those two brands?! Well pretty much everyone has heard of K&N, so it's a logical first point to call in (and probably has more stockists).
PS Fast car did a review/ test of diiferent cone filters for a fiesta ST (link below), they did find differences (but even the standard filter allowed the engines maximum flow rate), however, for different cars the designs may have different results (as they often use a set number of designs across cars), and as stated some cars standard airboxes are terrible, some a pretty good.