Skylake is likely to be another 5-10% improvement (the speed bump from the CTO CPU speed is likely to be in the same range as going from Haswell to Skylake - I fully expect the refurb 2.8 gHz Haswell 15" I have coming to beat all EXCEPT the CTO Skylake).
Polaris may be a more interesting upgrade, for things that like GPU performance. What people forget is that, outside of gaming, GPU performance isn't all that important for most things. There are certain functions in Photoshop and Lightroom that are GPU accelerated, but not all that many. There are a few more in video editing, and that's where it'll make the most difference in professional use. The user interface uses some GPU acceleration, but it's designed to work on relatively slow GPUs, because the majority of Macs have slow GPUs (the only real exceptions are certain top iMac configurations and the Mac Pro).
We aren't going to see a real change in disk speed on the new MBP - this one is NOT because Apple's being negligent, but because Apple is already using server-grade SSDs. The incredibly fast Samsung SSDs in the latest MBPs really don't ship as standard in much of anything else, short of some servers and workstations. There simply isn't anything faster than an SM951 that they could get their hands on.
The real question about the new machines is what we'll get OTHER than CPU/GPU specs? It seems certain that we'll get USB-C ports. If we get them as an ADDITION to other ports, they're a purely positive development. Even if we lose some ports, but not all of any single port type, they're probably mostly a positive. If we lose all of our regular USB ports, that makes the new machine a pain...
We may also get stuck with the MacBook keyboard, which, like USB-C only, is probably a "negative upgrade" for many people. To be fair, we could also get a variant on a butterfly keyboard with more travel, which would be a positive upgrade for many. I don't think anyone would turn down the OLED touch bar/Touch ID, if it came on an otherwise compelling upgrade, but I also think it's unlikely many people want it enough to put up with other annoyances to get it.
Other questions include storage capacity - it would be hard for Apple to exceed 1 TB with currently available drives, except by including an extra slot. Newegg simply doesn't show ANY PCI-E drive over 1 TB, except a few desktop cards (many or all of which are probably actually RAID configurations with two or more drive blades). The only sensible 2 TB configuration with what's shipping would be two of the same 1 TB blades they're currently using - and Apple almost never adds an extra slot!
32 GB RAM is much more likely - the chips exist, and it's easy if they want to do it. They've never made a soldered-RAM machine with >16GB, though - the 21.5" iMac, which is a Broadwell machine that would have easily supported 32 GB, is limited to 16. The 27" iMac is NOT limited to 16 GB, but that supports RAM expansion through standard SODIMMs.
The last real question is the screen - it could be the Retina Display we know (2.8k, very good sRGB but not wide gamut), it could be 4K, it could be wide-gamut (probably P3), or it could be both...