U N S C A L A B L E period.
Scalability happens, when performance grows in linear mode with hardware and even uses some synergy. This is one of mediawiki performance development goals. Make it distributed as much as possible. Make it as async as possible. Taming cheap horsepower is a challenge, but it helps project to work on low budget. There are guys in this project working with large systems, who are really skilled system administrators (though, maybe not second-generation ones :), and who know meanings of 9's after comma. Sure, you've got possibility to brainstorm and bring all kinds of ideas, but we aware, that those won't be accepted as absolute truth. People do not need prophets when critical thought is involved :) Though, any contribution to development is here highly appreciated.
Cheers, Domas