If your Free T4 level is high and your Free T3 level is low, you may have a conversion issue. If you are tested and this is the case, your doc "should" perscribe T3 (Cytomel). I say "should" because in general a T3 test doesn't always get run and lots of docs just aren't as up on it as they should be. Also, conversion issues are not all that common.
And to answer your question more specifically, yes, if you need T3 and you add it to your Levo, then yes, you will feel better and think more clearly.
I take it as I have pituitary issues and I cannot effectively convert T4 to T3.
It makes me feel much better and oddly, when I had horrid skin issues, it cleared up my skin a bit (even though the doc argued with me that it could not do that!). It also make me feel better mentally and made my temperature more normal.