I am a little cheap when it comes to spending xp so I usually research from same class since it usually has a lower research cost. In the case of the T-54, from the LTTB it is 225k xp, from Obj. 416 it is 150k xp, and from the T-44 it is 142k xp.
As for the LTTB being elited with research remaining. It has happened before. Last time was with the Tiger (P) when they intro-ed the new German HTs. Well, if you did not have the Maus already.
I included the LTTB in the OP just to show that tanks that were already in the game before update were still good, even if they moved tiers. That it seemed to be limited to just "new" tanks.