I have not ordered from the Butler a meal from a Specialty restaurant. I understand that there are others who have. I believe that your onboard account will reflect the charge from the restaurant even if the Butler delivers it.
The extra tip depends entirely on what you feel the Butler has done for you. I have tipped anywhere from $9 a day per person up to $15 or $20 pp depending on how much service we require. If I were having a party and using the Butler service, I would tip more for that day.
This is not really any kind of guideline, it is just what I do.
I also tip extra to the Concierge for the services she/he gives. I base that also on the amount of services we require.