I love good gas mileage and think the Focus does an excellent job of maximizing fuel economy with this car. They use some pretty neat technologies such as grill shutters, ADFSO (Active Deceleration Fuel Shut Off), electric power steering, "smart battery management", various engine mods, lots of aerodynamic treatment, low friction lubes and oils, low rolling resistance tires, etc. The best thing you can do to reach that potential is improve your driving style. To that end, they even provide optimal shift indicators and performance monitors you can take advantage of.
Other than driving style, keeping your tires properly aired and filters clean, there isn't much you can do in the way of improvement IMO.
If you read up about the snorkel removal, you'll find some seemed to improve performance in conjunction with other mods. Others reported a decrease in mileage. Maybe there was a reason Ford put it there in the first place?
As for the K&N, I used to be a fan, found it sounded cool but didn't really improve mileage any more than installing a clean filter. True, K&Ns admit more air, but since A/F ratio is closely governed and the stock filter operates nowhere near max air flow, the extra air is superfluous. You do get extra dirt in your engine though, dirtier oil and shorter engine life, if you're not planning on keeping the car for a long time, no-one will probably notice. Another (different forum) member who lives in AZ calls them "silica injectors"
In reality, filters are there to keep engines clean and minimize wear. It's hard to beat paper filters as this report indicates:
Other comparisons you can find on the internet show similar results.
If it were otherwise, wouldn't you think Ford (and others) would use them rather than spending hundreds per car on MPG upgrades?
Congratulations on your new car! You'll find lots of other great ideas to customize your car with on the forum!