FWIW, I just had my 3.5 yr old set of 850 CCA batteries checked my local shop with their electronic tester. They both tested "marginal" at ~680 CCA. They didn't strongly recommend that I changed them and offered me two options. $70 for a 700 CCA or $83 for 850 CCA. Of course I wouldn't touch the 700 CCA's. Both of them were Johnson Control brand that this shop private labeled.
Especially if you had access to a plug in for your block heater I don't think I would worry about the Optima. I added a 5 amp battery maintainer to my old Mercedes diesel that I plug in when I plug in the block heater which really seemed to help things as I started off with a fully charged battery every morning. I'm going to do the same thing to 7.3.
Have fun in Canada! Travis