I'm not sure it'd make much difference. He consumes imps only, so nothing would really change. You still have to spam shards for a bit before he comes out and the demons that are summoner still get the extra 15seconds.
Either way, something has to give with that ability. It costs too much relative to its impact right now, and keeping the costs as high as they are will necessitate a degree of over tuning that'd make the talent mandatory.