Adding performance tips to the documentation is a good idea! I'll make a note of that. For now, I'll say...
Well for mobile games I haven't gotten any mobile performance complaints yet, but knowing the backend of the code, things that could cause a lot of overhead are...
1. If you have a long string of text with an animated effect in it, this can cause extra CPU usage.
2. Setting new text or calling Rebuild() on a long string of text is probably the heaviest method in STM, even after being optimized several times... that said, as long as it's not happening under Update(), you shouldn't see anything.
3. The current outline/dropshadow shaders use a additional passes, so they cause 2x and 8x GPU usage than going without them, respectively. Still, not the worst when it comes down to it, as scary as 8x sounds...
Also, since it's an MMO make sure to look into things like object pooling for the text that pops up when you hit enemies! Having any object cached like that can save you from using Instantiate(), although I hear Unity is apparently more performant at that now, anyway.