In theory:
40km TT @ 30mph takes 48 minutes.
Cervelo S5 frame vs a "round tube" (non-aero) frame: 20 watts saved, or 82s. Aero wheels, let's say another 20 watts. That will save you 2 minutes and 45 seconds, out of 48 minutes (6.75% faster).
But that's only at that pace, and in optimal conditions. If you're riding at 15mph, drag is substantially lower, and you'll get almost no benefit. 25mph you'll get somewhere between 1/2 or 2/3 of the benefit. Real-world conditions (like crosswinds) will also change the actual results.
For climbing, let's say the entry-level bike is 22 pounds, and the über-bike is 16. You might get a 2-3% increase in speed with the lighter bike -- but again, if you're climbing slower than say 15mph, you won't have the aero benefits.
In terms of comfort, I'd expect it's a wash. Aero road bikes aren't made with comfort in mind.
An S5 with higher-end components will feel better (smoother, crisper etc) and will be more robust. Electronic shifting will require less maintenance.
An entry-level bike with Sora, a good saddle and good tires is $1000. With 105, let's say $1600. An S5 with Zipp 404 FC tubulars and Red (or Ultegra Di2) might cost $6500.