Since the retirement of the RS road bike with more "relaxed" geometry, Cervelo has unified its stack-reach schedule, creating a single geometry shared between the R and S series. This geometry is fairly close to the Trek H2 geometry, the more relaxed of the two geometries Trek supports. The logic used by Cervelo is that the vast majority of riders can be fit with a stem ranging from +6 degrees to -17 degrees using this more relaxed geometry.
And it's true, but many riders object to a bike like the S5 being touted as an "aero" frame had the longest head tubes. This was especially problematic when using the bike with full time trial bars. Indeed the S5 is sufficiently aero that with time trial bars, it's a competitive time trial/triathlon frame. So a lower stack provides more adjustability and allows more reach for the same stack and a shorter stem for most male riders (small women often benefit from a small reach). Riders claim to prefer the feel of stems in the 9-10 cm range over 12-14 cm (although I'm skeptical the difference can be perceived at the hoods or drops).
Here's the geometry data:
I plot it here, comparing to Trek:
So the new S5 is basically middle-of-the-road compared to Trek. A lot of bikes are in this range. So nothing extreme about the new S5.
One interpretation of the results is a 2 cm increase in spacers. But there's others. If you're running a -6 deg stem, which is typical, you can now up to the next frame size and shorten your stem by around 2 cm. If you're running a -17 deg stem you can probably swap to a -6 deg stem. So no hideous spacer stacks required here unless you want an fairly upright position or your arms are exceptionally short. But then you may want to stick with the S2-3, which is still an excellent bike.