It was mentioned some times.
Gen X #16: "Enormous ability"
Gen X #30: "A telepath of enormous power"
Gen X #35: "One of the most powerful telepaths on the planet"
Gen X #42: "One of the most powerful psi talents on the planet"
It was mainly intended as a student's book, so Banshee and Emma was never meant to show them up (same reason Emma and Scott were written below their average in New X-Men, to let the kids shine).
She did have some nice "feats" though.
Owning all the kids in a practice session (including Monet, see issue with psi-bolt)
Scanning the mind of X-Man over the Atlantic ocean, and later comfortably holding her own when he came to "get even".
Etc.
The one time she seemed to have difficulty was when she tried to probe her mind and got thrown out. Of course, this was not the Monet of today, but her twins who had somehow combined into one entity, so drawing conclusions of how M and Emma stack up from that might be dubious. Didn't Xavier also have problems with special minds like this? Rogue, for instance.
Also, probing a mind and encountering unexpected resistance is another thing entirely from a straight up telepathic battle it seems. Monet certainly did not hold her own there, once Emma got serious she was usually finished straightaway.
No, I've just recently re-read it myself so it's fresh in memory.
See above.
It was implied in some ways. Her easily taking out three X-Men despite Xavier's telepathic defenses and later capturing him off-screen. Also mentioned once in New Mutants #15 by Illyana.
"Possibly on par with Professor Xavier himself".
This is course just Illyana speculating, but it shows they had it in mind at the time.
You might want to re-read DPS, it was Mastermind who used a machine to project his illusions (his whole shtick, you know) into Jean's mind.
And, as a bonus, for those who believe she got powered up after Morrison's run. This is how Morrison himself saw it.
That is, Emma and Jean (before she got grew into the Phoenix power too much as happened slowly during his run), were pretty evenly matched.