Something like tidy plates threat plates might really help you out with AoE threat. Basically, it'll lite up the nameplates you DONT have aggro on, all you need to do is taunt to bring em back in line.
Also, in 5man heroics, unless your healer is weak, try to minimise time between pulls. if you can coast from one pull to the next with at least one mob still pounding on you, your vengeance won't drop, and therefore your threat should stay in line.
Also, you could try switching in some more DPS orientented gear, aiming for more haste. More haste = more hammer of the righteous/CS = more holy power.
In regards to holy prism; I started tanking with that, and found it was not adhereing to LoS. In the theramore scenario, it was pulling mobs from the floor above me. In karazhan it was pulling servants from the other side of a wall. I think it might be good in something like LFR and BGs, but for 5 mans, I would personally avoid it.
I would swap out one of those solid stam trinkets, too. probably the Brewfest one. See if you can steal something with expertise or haste.