You can't compare simulations to a real raid. Ever. Even the slightest difference from the actuall simulations throws everythin in the dumpster. For example, BiS normal gear. Which they obviously don't have. So I am afraid it is a valid argument
Yeah, you can get an estimate of what your spec is CAPABLE of performing. But, kicking someone out of a guild that is still doing normal (w/e boss it was) with the only reason being 'wrong spec' is just stupid. And is not a valid argument in any way.
Now, I don't know if it was the only reason. I highly doubt it. I would guess, that he is playing destruction and doing bad dps. But in that case changing spec won't help.