What is the impact of adding a floating IP to an active HA LTM pair?

One consideration to keep in mind:

When adding a floating IP address to an existing vlan that's processing traffic, traffic could immediately begin using that floating IP, so your backend resources could see a change in the IP address they are receiving traffic from. Rather than sourcing from the non-floating self IP, requests would begin sourcing from the floating self IP. Certainly a consideration when taking into account any firewall/access control mechanisms.