From personal experience I can say that Trijicon HD's will fit on a Shield
Installing them, however, is something of a pain in the you-know-where! I took a chunk out of my el-cheapo sight pusher trying to install the front sight. And the worst part was that I had heated the area beforehand with my heat gun to insure that it wasn't being held by Red Loctite... and was pushing the sight VERY slowly and carefully in the direction of the ejection port. Oh well... sight got installed and the sight pusher got repaired.