Only issue here is we really don't have many, if any, tanks willing or able to run these. We have like 2 tanks and maybe 2 off-spec ones in a 25 man guild so running Challenge modes for VP isn't as "easy" as they make it out to be (for VP).Assuming we're talking about an above-average, organized guild, Challenge dungeons are mathematically the fastest path to the VP cap. Of course the difficulty isn't comparable between LFR and Challenge Modes, but a serious raiding guild can go into an organized five-player setting and get to the VP cap faster, as opposed to gambling with a random LFR queue or facerolling LFR with a full guild clear.
You don't even have to be anywhere near a record-breaking time to be getting more VP/minute than in LFR.