Haven't heard of the shadowset, but I used P50 on my joint and it looks great. I used stringlines everywhere and planed/packed where necessary before plasterboarding the walls and ceilings though. Obviously dearer in man hours than a 90mm cove because of the setting involved, and the fact that you've got to use gappo here and there against the wall junction where the wall is a fraction out, but it looks really good IMO.
I'm going to use it against the polished floor as well, and possibly around the door jambs instead of an architrave.
edit: I just had a look at the
shadowset on the web, and I'm with your boss on that one. It looks like an expensive pain in the proverbial to me.
How are you going to get the same shadowline profile unless your using furring channel that stops short of the walls? You'd have to fix more furing channel than otherwise as well, or you'd have to pack the whole ceiling with strips of gyprock first. Or just mount it on top, but that profile doesn't appeal to me.

The idea is to keep it looking sleek and simple with no cornice.
Run a stringline if you want to keep the P50 straight, but you've just got to make sure that you don't use a crooked string line. Buy a new one if your old one's bent.
