I can assure you that Stobart do factor the Met vis, however EU OPS dictates that it can't be factored if the reported vis is less than 800m, so in effect, although the plates say 1,000, Stobart require a vis of 800m on the IOM.
If the Island wanted to invest any money at the airport, then upgrading to CAT II/III would be the best investment they could take, in the grand scheme of things it wouldn't cost that much and the benefits would be immense.
In Stobart's defence, if the schedule is disrupted through weather, or any other issues, they do all they can to get the schedule back and are the most committed.