Having used both versions, I'd say the P5 interface is slightly more sluggish than Pro Pack's, at least on my hardware. If you have a heavily-muscled system, you probably won't be troubled by it.

The Firefly renderer is usually slower than Poser 4's. I've gotten some speedy renders out of it, but it really depends upon what you're doing with the scene. If you have it tricked out with a lot of transparency, raytraced reflections and refraction, displacement, dynamic hair, polygon smoothing, heavy antialiasing, depth-of-field, motion blur, etc. and are using the maximum quality settings, then you'd best go out for coffee, because it'll understandably take a while. But the results are worth it, and at least we can say it's still faster than Bryce.

P5 includes the old P4 renderer, which is almost as fast as Pro Pack. It doesn't support all of Firefly's render options, of course, but if speed over quality is your concern, it's nice to have.

Other than the hosting plugins (3DS Max, Cinema 4DXL, Lightwave), Poser 5 has all of the functionality of Pro Pack. You wouldn't lose anything by upgrading.

Vue doesn't work directly with Poser 5, as you might have heard; it looks for the P4/PP executable before importing Poser files. Hopefully, e-on will address this in a future update for Vue, but since you already have Pro Pack, you should have no problems should you choose to upgrade to P5. Features specific to P5 (dynamic hair and cloth, procedural textures) won't import to Vue, but perhaps e-on will also add support for some of these in a future version.