Rebooting HA pair

MAC Masquerade means that all of the self IP addresses on the F5 unit will share a single MAC address, instead of having a unique MAC per VLAN. That greatly speeds up the MAC learning process on the upstream device as it only needs to learn a single MAC for the entire appliance to move within the CAM table. 

However, the MAC does change during a failover and the F5 will send out Gratuitous ARPs (GARPs) that notify adjacent devices to the L2 change. You can tune how fast the GARP flood starts and continues using database variables. 

For TCP connections, connection mirroring is required for seamless failover. This is how the standby device will know about established connections in order to continue those flows during a failover event. ICMP and UDP traffic will create a new flow upon the first packet, so you should not see interruption for the stateless protocols. Thus for true HA failover, enable connection mirroring. The "system degradation" isn't really a factor but use a dedicated interface for HA (config sync and mirroring) to keep that overhead away from your data interfaces if you're concerned.