@the black fox not that I could tell, the 40-150mm f/2.8 is lightning fast, and fitting a TC to it seems to make no difference in that regard, but then I'm shooting cars and bikes, they're not so twitchy!!
I don't want to sound too disparaging of the Panasonic 100-400mm, I didn't have it for long, but I just couldn't get on with it, zoom ring too stiff, zoom action too long (the 40-150mm f/2.8 zooms fully in / out with a quarter turn), and because it extends when zooming it changes the balance of the camera as well as the aperture etc. But then I see other people doing great things with it, so I'm sure it's just me.
The 40-150 f/2.8 is such an amazing lens, and the internal zoom + constant aperture are under-rated benefits in my opinion, so fitting a TC to it for me is a no-brainer, if there's a degradation in image quality I (with my old eyes) can't see it, and if there's a slight loss of focus performance I (with my old reflexes) don't notice it.