Here's a test by Tour Magazine from 2015 which compared the Cervelo S2 directly with the Cervelo S5, and included aero data on 14 other aero bikes:
https://www.tour-magazin.de/raeder/r...al/a34659.html
This is from around the time that bikes reached the era of "peak earo". Since then, there have been little improvements in aerodynamics, and more emphasis on maintaining aero performance with new component technologies (like disc brakes). Interestingly, some independent wind tunnel testing has suggested that aero performance has taken a slight jog backward with the addition of disc brakes to aero bikes. Trek has been upfront about admitting that their disc brake aero bike is not as aero as their rim brake aero bike, and if you read between the lines of Cervelo's white paper on their disc brake S5, you can see that the disc brakes also had an aero penalty to this bike as well.
Why have S2, S3 and S5 models? Primarily to meet price points. Independent testing has shown that there are differences in aero drag and weight between these bikes, and of course there are differences in component spec. as well. The S2 will have most of the aero benefits of the S5, but at only about half the cost. The S3 is in the middle.