What if Teemo's ult had Kassadin-like ult conditions?

Mana cost increases based on the number shrooms Teemo currently has placed on the map up to a cap. The cost increase remains until a shroom either expires or is destroyed/stepped on, after which the cost lowers based the number of shrooms currently on the map.

You don't have to make the cost double per shroom of course. If his ult were done this way, would it be fair to put some power back into his ult to compensate for the Mana cost increase per shroom? Maybe even reduce the CD?

Teemo mains?