Finally had some free time and went to check on the stacking effects of the newly introduced weapon proc effects. To my surprised, every single one stacked.
Cursespewing and Improved Cursespewing yield a -8 penatly.
Destruction and Improved Desctruction yield -12 ac.
Shattermantle and Improved Shattermantle yield -9 SR, despite the text on the effects.
While only the Shattermantle combo explicitly says it shouldnt be stacking (and it is, so Im assuming at least that one is not working correctly) it seemed odd enough I wanted to ask for confirmation. Granted its not terribly easy to always land and sustain all these effects on a target, but its still something that seems a bit odd.