I would think the fps hit from a second bus would be negligible considering surrounding airport ground traffic at most if not all 3rd party airports.
Ground traffic in airports is not usually made with the same kind of detail of the GSX bus, which is a fairly high poly model with full PBR, so it's likely equal to about 10 vehicles of the kind you usually in 3rd party airports. This because it was DESIGNED to be shown "alone". And, it's also full of people, when arriving. Sure, people with less detail, but still additional polygons. Again, nothing is "free" on fps.
As for time savings, yesterday at FDST PHNL Gate C1 I had 420 passengers on 5 busses take 25 minutes to load (passenger timing very dense) onto a PMDG 744. Almost 10 of those minutes was spent waiting for buses 2 thru 5.
As I've said, it depends on the airport. I never claimed that PHNL, which was made 10 years ago, has an AFCAD made to take into account GSX, so it might not have vehicle parking spots placed in the best places, we'll look into this.