There is no measurable power increase with using the football cover or it being open.
Having a cover, like the football type will add some protection for the filter but not being as exposed to the environment.
I like the cheap round cover with a slot at the bottom. It looks good and allows plenty of air in, plus you get protection from the rain.
I do like the one you picked. Looks great. I would consider painting the black blue to match the color of the bike. But that is me.

 

I have been thinking about having it painted, the shop that painted the bike has some paint left. I am trying to decide what else to have painted but with the starter and ring gear fix it may have to wait a little while.

 

I read a bunch here before buying my S&S stealth, so what I did before buying was I monitored intake air temps for a week or so prior on my fp3. Then again after the install. I'm seeing pretty consistent 5-8 degree cooler air intake temps and the temps drop quicker when riding after being in traffic too.
I notice no performance changes at all though. I also run with no covers.

 

I have noticed the bike runs a little cooler. I am working on a couple different ac covers, easy change and makes the bike look different with one bolt.

 

When I installed my VH pipes my FXDC Dyna ran a little rich. I would find some carbon residue on the swing arm. I went to a K&N filter and solved the problem keeping the OEM air cover . Easy fix, maybe 1/2 a beer. I do not think you will see a difference with the cover off. You will have to watch for rain that's for sure!