Sounds like you are doing everything correctly. It is totally normal to get an increase in airtime utilization on 5GHz when cutting the channel width in half to 40MHz. Now, 802.11ac clients may take up to twice as long to transmit the same data on a 40MHz channel as on an 80MHz channel.
Of course, you are doing this for a good reason -- you want to deploy more APs to increase capacity. 40MHz is a great setting for that.
I would definitely not recommend reducing the number of AP's. In the future, you might need/want more AP's if your number of clients or bandwidth requirements grow. Or maybe one day you'll find that a good percentage of your clients support Wave 2 and hence your whole network will benefit from MU-MIMO. That might be a good reason to introduce R710's in your denser areas.
Until then, I think you are well provisioned. I wish I could have your kind of utilization! Due to periodic machine backups on my network, my 5GHz utilization peaks at around 30-50%. But even during that kind of heavy usage, my clients still perform great, and there's no noticeable loss in performance.
It's great that you over provision your network and have invested in the best wifi solution on the market. I'm sure your users appreciate it, even if they don't know how much worse their wifi could be!