Infernal Cinders is RPPM (as are most trinkets), so your attack speed is irrelevant for the most part -- Arms will have a higher chance on each hit to trigger it, resulting in the same average number of procs compared to any other spec.
First thing to do would be to ensure that it is doing the proper damage per hit... does it look off to you? A quick look seems to match the latest spell data that I have. The proc rate looks correct to me as well... 10 rppm modified by haste.
I also have updated arms warrior gear rankings going live on the site this weekend -- could see how that changes things as well.