Upgrading LTM from 10.x to 11.x

There shouldn't be any problem with your plan of action.

Your true outage window should be very small. My team and I just performed several upgrade in the last month, and as long as the upgrade goes smooth, the only outage you should see is during the failover to Active. For us that was a matter of seconds. Just make sure that you validate that items in your config will transfer properly. e.g. iRule syntax