Hey, I just upgraded my chest piece to tier 10 ilvl251 and it finished off my 4pc bonus. I had an ilvl 245 crafted chest in titanium razorplate.
The old chest piece had one more gem slot than my new one, which for my gear turned into an extra 20 armor penetration. After adding everything up, I lose a nominal amount of arp but I gain crit and str. These gains outweigh the lost arp from a dps standpoint but just barely. The t10 bonus, however, makes this choice a no brainer as it adds at least 500dps on the top of my 25 man dps.
Wow heroes ranks both of these chest pieces the same. I know you're taking into account the tier 10 bonus, but apparently that bonus' score doesn't outweigh 1 gem? My wh score did not change 1 point after this upgrade.