Before 7.5, tier 5's were quantifiably the most reliable xp and credit grinders once elited. Even if destroyed in a losing match they still turned a profit. Their maximum realistic earning potential was less than higher tiers but the effective minimum earnings pretty much always left the player in a better position than where they started. This is the information as related to me by other players. No idea how true it is since I always used a combination of Lowe, Slugger, and JgPz4 for free xp (until I elited the 13 90, which is a great xp mill even though it usually loses money now.) Based on what I've seen with the ELC, tier 5 is still pretty good for earning credits at the middle tier.
In other news I have just unlocked the 100mm for the Lorraine.
/lacks credits to actually buy it