But I would think it's possible to get the seal too tight. If you increased drag but didn't increase the effectiveness of the seal, there would be a loss of velocity. I don't think wear would be an issue, since these seals can be rejuvenated so easily.
But this is mostly just thinking out loud on my part. You might ask in the regular yellow forum or over on the dianawerks collective. There are a lot of guys who have lots of experience with leather-sealed european guns that would know.