Jerry, thanks for the info. Has anyone fitted a K&N HD 0910 to the SESG2, and has it made any difference, or is the stock breather pretty much the same? 
The stock filter is a high flow filter. Mine will likely get replaced by a K&N at some point in time, but I will follow the owners manual instructions for cleaning the stock one in the meantime, and will use it until it shows visible signs of deterioration. IMO, there would be little, if any, difference in performance between the two filters. The only way to know for sure is to do back to back Dyno runs with the same bike, under exactly the same conditions (air temp, humidity, etc). If the bike was tuned with the stock filter and then fitted with a K&N, there would be no need to tune again.
In the bad old days, before HD started putting the high flow air filters on the CVO bikes, there was definitely a need to change out the entire air box, backplate, and filter...the stock assembly was very restrictive. That is still true for the "regular" HD bikes. So on those bikes, you could go with the SE Air Cleaner "kit", Zippers, Arlen Ness, Doherty, etc. Or you could install the Ventilator A/C like you already have on your bike...I personally like the looks of the Ventilator, but to each his/her own.
Keep in mind that even a K&N filter only flows maximum air when it is freshly washed and oiled...it's the nature of the material used. A fiberglass filter will actually flow more for a longer period of time. However, the K&N gauze type filter will hold up longer over time and repeated cleanings. I have no data to support this, but my guess is that the stock filter will hold up through a few service intervals just fine, if cleaned properly. But if you've got $60 that's burning a hole in your pocket, then the K&N would be a good long term filter to purchase.