For some reason, I would more eagerly participate in a Frontal Assault on Stormwind rather than a sneak-in type of deal.

While your idea is one I've used in a situation or two (sneaking a small group into an Alliance location and using Warlocks to port in the rest of the crew), just something about assaulting the gates of Stormwind makes me feel that it would be more impressive, even if we fail to take over more than 1 district.