Tenacity from items should stack additively with Swiftness mastery

Right now, Tenacity from items (, ,) stack only additively with Swiftness mastery. If you click a link on the page I had linked, you'll see that +Swiftness mastery gives only 30% tenacity (instead of 40), and this lowered effectiveness (multiplicative stacking) is also true for and interaction with the mastery. This means that having 18 mastery points into resolve tree is has less of a value against heavy AP teams (having a lot of cc), since going any of these 3 items doesn't give the intended value you pay for. Juggernauts suffer from this poor interaction the most, as most of them go Grasp of the Undying as a keystone. This shows in competitive play the most - none if them is meta right now, except who is a hybrid tank (with ) more than a Juggernaut, as he deals burst damage more than sustained.

TL;DR: Multiplicative stacking between items is fine, but its interaction with Swiftness mastery should be changed to be additively.