LOL, a ICON/929 firmware on a 928 ? Good luck with that, first of all it would not install(locked down by certs via product number), second of all, hardware is different, display, chipset, CPU. almost everything is different (maybe a few chips are the same)
IF you could get it to actually install (and it wont), you would BRICK your 928..and turn it into a nice paperweight...A worthless device that even Nokia tools might not be able to bring back...
Not a good idea...Unless you could make a custom rom, with the correct drivers in the image but, you need it to be certified or it wont be able to install....and right now, that is impossible, even the expert hackers over at XDA have had no luck...