Perhaps the fact that most of the Elantrians we see are brilliant is not a cause for their change but rather possibly an effect of the Shaod. Maybe there are three aspects to the transformation:
1) Physical - They are changed, white skin, beautiful.
2) Mental - They are able to put together complex problems and solve the math like magic system.
3) Spiritual - They are attuned somehow to being able to access the Dor.
This could possibly fit with some of what we see in Warbreaker too. The Returned are definitely changed physically, they are also changed spiritually by having one super powerful breath, maybe they are changed mentally and we just don't know it?
PS. First post, been reading the board for a while and this just occurred to me, so here is my hat in the ring.
Possible, but most of the achievements I cited existed
before they became Elantrians. They didn't lose them even with the underpowered transformation, but if this theory has any merit, the seeds were already present before.
Also, I believe that at one point, Lightsong comments that Returning does not make one any smarter than a regular human. We don't know if he is right or not, of course, but information on the subject is sparse. As noted, though, the Returned are definitely effected mentally---they lose their memory, but not, apparently, their capacity.