I think you're missing the point in the definition of 'not stable'. The word suggests that the software causes unexpected and random bugs. However in this context it's used to mean 'not proven to be without bugs'. In practice it means the software is 99% stable, but since improvements are still coming up regularly, it's not acceptable for high-load, popular websites. I think the core is stable enough, it's mainly the modules that still have some bugs. Personally I think Siena is already stable enough for 90% of websites.
Aparently the problem with the load time on cotonti.com is caused by the loading of resources, not the page creation time. This means it has nothing to do with the engine. Siena or Genoa, it won't make much of a difference.