I can get around 28fps at Aerosoft Heathrow with same number of Ai.
You cannot possibly know if the AI impact is the same, even at the "same number", because this doesn't take into account which models they use (their polygon count), and how many *DIFFERENT* liveries are loaded and how many *DIFFERENT* models there are at any given time. The more different stuff is loaded, the more the impact.
That's why it's always wrong to test a scenery with AI enabled. If you want to know if the scenery is a problem, always test with 0% and NO weather.