Have a 2010 Ultra,and I have to take them off during the summer.Not enough air flow over the engine esp the primary. Harley.offer's a quick change kit,but pretty easy to take off like they are.
My lowers were on for 25 miles and have never been back on.
I may do some experimenting in the fall to check the difference in wind, wind noise, and buffeting. I prefer he looks without, but if it is an improvement I may start using them fall/winter.
I haven't ridden bike w/lowers so have no frame of reference, but I was amazed at the difference fork deflectors (fangs) made. Little warmer on the legs though, and kinda ugly IMO, but might be an alternative.
I have been looking for lowers for mine, the prior owner threw them out years ago, I bought the Harley soft lowers and use them when its cold or raining. I think the hard lowers look better so I will probably eventually buy some and have them painted to match.
I run the Chilhowee indian leather removable lowers in the winter. I'd love some hard lowers but I changed my engine guard out for one of the burly 1.5" type and I understand that they won't fit.