You can totally change to armor from robes, without worrying about threat. Effect of armor type on threat is massively overrated. Light armor draws 5 threat more than robes or being in your small clothes, and medium/heavy/massive draws 5 more; and this is added only once upon being perceived. 5-10 extra threat is negligible, as in tending to zero.
The thing is even if you are streaking across the battlefield, if you are dealing crapton of damage, they are gonna come after you. Don't blame your armor, blame Alistair (or the tank) or yourself instead.