Impossible to say since glossy vs. matte depends so much on user preferences and environment, and for my money glossy is superior (esp. with the iMac's rather effective anti-reflection coating which I assume the SD shares) and matte is a compromise if you're stuck with a light or window immediately behind you.Now of course there is an argument to be made that the ViewFinity S9, having a matte display, is really in competition with the $1899 Nano-Texture ASD, not the $1599 "standard" (glossy) version, and should slot above the glossy LG as well.
Anyway, to drag this thread back approximately on track - a 3rd-party 5k display would have to come in at under $1000 to provide $1800 4k iMac replacement (allowing $100 for 3rd party keyboard and mouse - personally I wouldn't dream of paying $250 for a Tragic Mouse and Keyboard combo).