While I'm not entirely sure how SS handles its current cache. One thing I'm very sure is that the caching is not effective enough for me and anyone else who may be thinking of creating big sites using SS. The only solution to this problem is if output buffer caching can be built into the system.
To be honest I would suggest this to be done earlier rather than later - as SS becomes more complex - when it comes to implementing output buffer cache - life would be hell to try and ensure every module that cannot be cached (for example due to permission check before page is displayed) have some sort of no-cache flag.
I personally see output buffer caching as an essential part of any PHP powered websites as not only does it let your website have the performance of a static HTML site - it also reduces server load from both the web server AND database server by a huge amount (>500%).
If someone could shed some light on how the current system + architecture works - I may even look into building this myself.