I've a concern that servers that may not want to allow for members to boost have no way to disable it.
There should be some kind of system that allows the admins or mods of a server to enable or disable the function. And perhaps further, add something that'll allow the admins or mods (assuming they allow boosting) to allow if the special role will show up or not.
And any servers currently that have been boosted should be allowed to moderate those boosts. If there are servers that want to undo the boost, it should be allowed for the admins/ mods of the server to remove it. And if the member that did the boosting leaves the server the boost should be either automatically undone or again the admin/ mods should have the option to undo the boost.
There can be people that will definitely try to take advantage of the 'special' roles they get as a booster who shouldn't be allowed to do so.