In fact, I think Touma has been trying to keep a low profile on his ability. I'm not too sure that besides loli teacher, anyone know of Imagine Breaker or all of what he was able to do in the index arc. He probably discovered the extent of his own power along the way.
Why am I writing this?
Because Mikoto was surprised he's able to resist her power.
More importantly, Kuroko was surprised she couldn't teleport with him, when she should have some data in the database regarding his power.
There's also the fact that some people in the background know about touma, like the guy in that strange room talking to the esper using magic who works as a spy. Knowing that, they use him for some purposes and ultimate goal we do not know of for the moment.
To summarize:
Yes the measurement system is flawed for Touma, and maybe other students.
Touma uses that flaw to keep low profile
Some group know about Touma and use that.
Now, it doesn't help us knowing much more.
Also, it remains a bit out of subject in the Railgun Arc (at least for the moment)
The only thing is that Touma could also be a key to anihilate both Magic and Esper powers from the Index+Railgun world.