Hi,
I have been using a K & N filter in my 05 1200s for about 6 months now (or for about 10k miles). I really can't say that it's any better or worse than the stock except that you can reuse it for the life of the bike which could save a lot of money. The best price I could find on it including the cleaning kit was from Chaprall Motorsports.
Also, I did take out the snorkel from the airbox.
But as for warm up, overall power, and any kind of performance difference I can't detect any difference from just using the stock filter.
As for the spark plugs the dealer told me to change them every 9k miles.
Lucas