I ordered a 15" MacBook Pro, but then saw the rumor that new models might be introduced. I kind of need the computer before June 1, when I go on a long trip. But if the new model is (speculated) to be a lot better, I could perhaps wait until I come back. As far as I can read, probably it will just be a CPU upgrade? Would that make a huge difference?
One advice I think is really good, I got it a few years ago: if you need a computer (or any device) now, don't wait.
With that said - if you can wait, wait. It's not just that the new computers will be a bit better, price cuts may also happen. I mean, maybe the new models are, actually, $100 lower.
But to answer your question - no, the new MBPs probably won't be much better. The Kaby Lake will be slightly faster (probably not noticeable) and the battery life will be slightly better. Also, it may come with AMD 500 series GPUs which will, most likely, just be Polaris rebrands (with maybe slightly higher clock speeds). AMD is known for these rebrands. It almost certainly won't be a Vega GPU. I obviously don't know, but I think you will see really minor speed gains. Apple will most likely just do a small spec bump to avoid criticism of not having the latest chips. It will probably not come with the ARM secondary chip for notifications during sleep and stuff like that.
The 2018 upgrade will be more interesting in that regard.
Still, again, if you can wait - you may end up with a better computer, even if only slightly. Then again, if you need it NOW, heck - if you just WANT it NOW - get it.