The only issue that's been biting me when using ActiveModel::Serializers and AngularJS is that we must manually return the json with the root: false option set, otherwise the AngularJS $resource provider doesn't work correctly.
It's difficult to benchmark, to be honest: Rails apps depend on so many things, there are too many variables. I'd love to hear from some people who have tried it in their apps though!
I'm more interested in it as a place to explore best practices and work on useful things around names, than the speed aspects.