Boost doesn't have access to Sprints 4g, the best you can get on a flashed to Boost Evo would be EvDo RevA(3g)...UNLESS...you know someone who is currently signed on with Sprint, who is gracious enough to let you use their personal wimax numbers. You could then program those into your Evo on Boost and piggyback on their 4g data. Otherwise, 3g is the best you can get on Boost , at the moment anyways.
As far as flashing Virgin mobile, I'm a Boost guy, so I wouldn't know about that.