Depends on how many and what type of AI packages you have installed, along with the type of AFCAD (parking/gates) files you have installed. No matter what, many GA fields are probably going to be a lot less busy in FS than in real life, while your major international hubs may actually be busier in FS than real life.
I personally use the following:
Airlines:
World of AI
Military:
Not much, but some installed MAIW packages.
Civil/GA:
World of AI general aviation files.
HTAI Cessna payware.
I also try to use non-default airport scenery when possible that enhances the number of parking spots at airports. Especially noticable is the major lack of parking at default FS international airports, such as KJFK and KBOS. (I now use the excellent FSDreamteam KJFK payware to solve part of that issue).
I run my aircraft traffic at 100% in FSX.
Airport vehicles at minimum.
Road traffic around 6%.
Large boats at 24% (depends on where I fly, sometimes it goes up in areas near large bodies of water.)
Leisure boats around 26-30%. (Again, depends on my flying location.) Both boat variables vary dramatically depending on where and what I am flying.
I also use EditVoicePack with FSX and the option to speed up the speech slightly. Sounds better to me, and can have benefits to the AI flow as well.
Mr. Gibson, pleasure to have you on these forums. I have not looked into this extensively, but are your excellent classic propliners & jetliners traffic packages compatible with FSX SP2?