First, I don't think you lose the mana seed trait. If you do, that's the first I've ever heard of it.
Unless you're referring to being unable to see it in the list of traits. It's a well-known issue that the trait list does not scroll, so once you have more than the dozen or so slots filled, you can't see the others. They're still there, just not visible.
Personally, I haven't noticed a great deal of difference with having my spellcasters boogre-ized or not. With a high enough sorcery and spell school level, you cast successfully anyway.
And I know that Willow has completed a game or two while Boogre-ized, and she's always looking for company.
Try it. There are no right or wrong answers in W&W, only experiences...