I'm getting more and more tempted to vote for it, but there's actually an additional drawback except for those related to the cost. Having three Light Lord Magisters involved in the Waystone Project would quite overshadow the Grey Order and, more importantly, opens us up for a Light takeover, at least when it comes to control over the Empire side of the project.
Although, now that I think about it, I don't even know how much of a problem that would be for me personally. More politics and pressure from above in the Quest aren't drawbacks to me. And anyway, we could balance the Light Order out by winning an equal contribution from the Jades and by having a secure hold on all contributions by Dwarves (or others who only joined due to our personal reputation).