Agreed that most of these would be good components of an overall browser benchmark, but the linked benchmarks are JavaScript benchmarks only, so things like browser start-up time and chrome dimensions wouldn't apply.
When users think "fast", they think "fast" as in "fast", not "JM fast" or "TM fast". It would be better for "arewefastyet.com" to address "fast" from the point of view of the user rather than from the point of view of the Javascript engine developer.
As I understand it, arewefastyet.com was created as more of an internal/community dashboard specifically for the JIT team at Mozilla rather than to reflect on the browser as a whole to the userbase.