I personally like Holy Nova alot aswel, It's a very good spell to get your DMF trinket to procc in start of a fight, even using rank1 of it. also the more healers you bring to raids, the more powerful it becomes due all the flash heal/flash of light sniping. At least in our raids.
Also about the best priest specc. I think it's very situational and you have to communicate with the other priests in your guild. Ultimate goal is to have 2-3, not more specc for imp renew. when more people have it, I dont think it becomes effective anymore. Also make sure not everybody is throwing out renew because it will make other priests less effective.
Recently I managed to get full t2, together with hazzarah charm of healing and I see huge potentials with it. I kinda stopped using flash heals completely, instead having gh1, gh3 and heal3 as main healing spells. It will be alot of overhealing but the raw heal is insane without having to worry about ooming. I could also see a potential of a imp renew specc, with gh spam during spikes, renew during downtime of the trinket with a gh1 here and there.
Overall I would say its bad to let all priests specc "the best priest specc" Everything is situational and try to communicate with the other priests, their playstyle and what they like.