Actually when the IAR was introduced, both the 81a and 81c could carry both all three bombs.
That was until some w@nker that pretended he knows it all, sent some whrong info to oleg and ever since that patch, the Iar81 was reduced to either one 250kg in the center, or a pair of 50kg bombs on the wings. But not all three boms as seen in most pics of loaded iar81's. In the same patch, the Iar also got the wrong allied constant speed prop.
Your idea to limit the iar81 loadout to a single mission type, like the Ploiesti defense would negate the rest of the jobs it did on the front line. The iar81 was used both against Russians and Germans. On the front line the plane was used as fighter bomber in many ocasions. It wasn't the dedicated dive bomber, what some planes were suposed to be, because the Romanians got Stuka from Germany, but a fighter/fighter bomber.
When you don't load bombs I don't think the drag created by the rack is taken into account. I may be wrong tho. I didnMt play the old il2 for a while.