Our guild has never really focus geared people or roles. While we aren't world class or anything (can't say its a goal either) we manage to kill all bosses on mythic in HM and pretty much trended that way since BC.
If you have the skills and leadership who has what gear matters a whole lot less as long as they aren't dreadfully behind the curve. Unless you are going into content so under-geared (like you could have been doing mythic HM in mostly blues for example) that it makes that big of difference. This level of play is very, very rare for the very, very bleeding edge. Also considering now you can run both heroic and mythic is a week ALONG with normal getting set bonus and the like is going to be pretty freaking easy. I would say everyone within 3-4 weeks will be mostly bonuses out.