Advice - don't assume too much. This game of math (as any other game ofc)
Here is the right algorithm of "assuming" in present WoW:
1. Find the "bad" items - The "bad" items in this tier are the hands, that give give crit/haste, and the legs, that give crit/multistrike. The hc hands give 150 crit and 122 haste and the hc legs give 163 crit and 199 multistrike.
2. Find the "better" items - the arcane offpieces in BRF for the according slots are from Gruul and Iron Maidens. They give as it goes:
- gruul hands - 126 haste and 148 mastery
- maidens legs - 194 crit and 178 mastery
3. Do the math yourself:
- with tier hands and maidens legs you will get 344 crit, 122 haste amd 178 mastery.
- with gruul hands and tier legs you will get 126 haste, 148 mastery, 199 multistrike and 163 crit.
4. Check your stats ranking and see what will be your most beneficial combination.
As you probably know crit is arcane's "worse" stat, so i think you see what combination will be better right?
P.S. see i didn't assumed anything. Just a bit hard cold math :P