Need advice, telling a mage he's badFollow

I hope he isn't the class leader. Go through the person above them, let them know what your role was in the previous guild (don't go on bragging though) but let the ranged dps/class leader know that they could be doing better and what they can do to help him or what you did to help them. Depending on the type of guild it is, people still care about what others have to say about how they perform (as long as it is done tactfully). Make sure to explain to them (gently) what they are doing wrong, why it is wrong, and how to correct it. When they see their dps raise by a large amount they will be grateful, as will the class leader.

What, is it supposed to be a surprise or laugh worthy if a female is a good player? :-p I'm female and I'm awesome. Okay, awesome might be a bit strong, but I'm definitely above average.

Just be tactful and talk to the other mage and the raid/guild leader. It might be better coming from them than the newbie. :-)

politely point it out to the GM. tell him your situation and how you could help. Ask the GM what they recommend - then do it.

Tell him directly non publicly

You won't get any credit from the other players for being super leet but you also wont cause any drama

If he refuses to take your advise just tell him the benefits again, I'm sure if you came from a hardcore guild he will respect your opinion on the mage class

At that point if he still refuses to take your advise with the old "this works for me" just crush him week after week on the meters until he breaks down and asks you

I would say leave it alone for now. I think the closest you mentioned to his actual dps is that it's just plain awful but even that reference was a little vague. You say Rawr sims him as being capable of 6k dps. Is that with or without raid buffs? If he's close to 6k dps (ie. he's making use of what he's got, even if what he's got is sub-optimal) to the tune of say...5-5.5k dps...he's an asset in ToC10/25 and, while he would ideally want to be putting out more in ToGC25 he can still contribute. Our damage spread in ToGC25 (depending on the encounter) is ~5k at the bottom end up to 8k at the top end (excluding gimmicks).

If it's a case where his gear sims at 6k dps and he's doing 3-4k on stand up fights like Gormok or Jaraxus where there's occasionally movement required but not constant movement, then yes there is definitely an issue and tweaking his spec/gear for an extra 1k peak potential dps is only the start of bringing him up to speed.

It gets tricky, however, in a casual guild because quite frequently the leadership isn't interested in working with people to improve. They just take whomever, run the raids, and if the dps is too low to get the job done they just declare the content too hard and go back to farming what they can clear. Ideally you would wait and see what the guild leadership has to say about low dps in raids. If they make the comment that some people need to be putting out substantially more dps, that's your opening after the raid to approach one of the officers and suggest to them that there are some things the mage in question could be doing differently in order to post larger numbers.

Without that opening, stepping forward as the new guy to offer a critique of someone else in the guild is a risky situation. I would say definitely do not approach the mage directly about this, and if you see an opening to approach an officer try not to go in with any expectations around results. At the end of the day it can't be worse than the loot whoring mage in Ulduar10 in the guild I ran with before I transferred servers. 1.1k dps averaged over 5 bosses.