We do sell them for the T3 TD, it's the same filter in a almost identical housing, they do make a difference, albeit a small one.
Also, it depends where you buy your filters depending on how long it takes to pay for itself.
I based the 4 filter calculation on what I sell Mann Hummel filters here in the UK for against what I sell a K&N in the UK for.
Seems you pay more in Germany.