Mentalists have Mind Tongue which is similar. The main differences are:
1) Mind Tongue requires concentration and ends without it.
I think changing the spell to 10 min/lvl (C) would be a small boost (+1 level?). 10 min/lvl (without a concentration requirement) is a bigger boost, but given that it only allows communication and nothing else, I don't think a level adjustment of more than +1 or +2 levels is necessary.
2) Mind Tongue has a higher initiation range but is always limited by that range, while Mindlink becomes unlimited after the link is established.
In practice, I don't think there is much of a balance concern here, since you have to start within range. It's a boost but unless you are doing a lot of teleporting it won't be a big one.
3) Mindlink with additional power can create a connection to multiple targets (possibly so they can all communicate together? ambiguous)
That's fun, and I think would work better at the table anyway.
You could replace all the Mind Speech spells -- and maybe Mentalist Tongue too -- with a scalable version. Pick X number of options from this list: 1) communication with non-mentalism realm recipients, 2) increase range (may be selected multiple times), 3) maintain link even if target goes out of range, 4) add one recipient (may be selected multiple times), 5) communication does not require concentration, duration becomes 10 min/lvl.