You have it right regarding Custom Applications versus FDQN Domain Objects.
Some discussion on this from a while back: https://community.checkpoint.com/t5/General-Topics/The-difference-between-checkpoint-creation-domain...
If you're not already running one of the Threat Prevention Blades or App Control/URLF, then there will be a performance hit enabling App Control and/or URLF to implement a Custom Application/Site.
Otherwise, shouldn't be much of a difference.
Where HTTPS Inspection comes into play is for the Custom Application/Site...to parse the Host: header or the URL.