I'm in Montreal. If I understand, most of the speed boost I might feel will be EC2 instances and CloudFront stuff. Will it have any effect on S3's latency ?
It won't affect latency to any bucket in any other region. But if you create an S3 bucket in Montreal then you should have excellent latency to that.
As far as I understand, it will have a positive effect on S3's latency, for straightforward reasons. But to what extent? Maybe some BOE calculations might help... Throwing out a guess, the gains probably won't be too extreme.
EDIT: Of course this is dependent on what region you're currently in!