Ok I decided to run some test today to see any real speed differences took me 3-4 hours, this time I ignored VMs and straight to solid hardware.
Had an old Ocz 3 ssd with 500mb read/write speeds so used the same SSD for all Win 8.1 installations. I used a stop watch and also checked how many times the windows circle sandtimer loops!
Installed Windows 8.1 core (64bit) takes roughly
15 minutes to install with
17-18gig fully installed (no tweeks) I would imagine the Pro edition would be similar, same speeds and size.
Then I installed Windows 8.1 embedded industry Enterprise (64bit) MSDN copy it takes
3 minutes to install and
16gig fully installed (no tweeks). I found this strange since when I tried it on a VM somehow I got it down to 8gig perhaps I ran a tweek or 2

. To confirm this I formatted the SSD and reloaded it since it only takes 3 minutes to install why not, again it shows 16gig something.
Now onto general performance, Windows 8.1 Core 64bit takes a good 3 circles timer loops to get into desktop, Windows 8.1 embedded industrial about 2.5 circle timer loops.
Installing software, I tried comodo firewall, Avira and Libre office again pretty much similar speeds nothing was faster all about the same.
Overall thoughts: Not much difference in speeds with desktop loading and considering Windows 8 already loads in 5-10 secs to desktop not much difference in Core/Pro or embedded version here. General usage and installation of apps not much difference either. Industrial Embedded is not any faster then normal Win 8.1 Core/Pro and once you install firewall/AV and Device Drivers, it always slows down a tad. I installed the same firewall/AV and several apps each time to make sure.
The main difference is however is the installation time the embedded took me 3-4 minutes to install where as my Win 8.1 Core OEM copy took 14-15 minutes to install!
All Windows 8.1 copies be it Core, Pro, Embedded enterprise/pro editions I tried ended up roughly with 16-18gig installation on hdd. You may be able to knock off a few gig with disabling pagefile/hibernation and tweeking/cleaning Windows junk files though.
Worth changing over? Not really if it started up much quicker or installed general games/software quicker I would move over today, if I was installing an os though you can't knock the 3-4 minutes it takes to install the embedded edition. So if your looking for a speed difference I would say not.
Since it takes 3-4 mins install time others can try also.