from what ive seen, the Dark Reaper exarch is about the only one who ever does anything in the unit so you might consider dropping the other two dudes if they remain in your army. I would also advise against the warwalkers. To me they only seem good at outflanking with scatter lasers to ensure side shots. If you are looking to pop transports early on, then you have to set them up in your deployment zone and they are just too easy to kill. Especially when your whole army is getting cover saves, your opponent gets a lot more bang for their buck by shooting them, so they are a prime target.