To me, there is very little change in the sound. I've put Termis on all my previous Ducati's (Multis and Hypers) and the sound improvement has been awesome once the baffle was removed. There is no removable baffle in the 950/1200E Termi. When you look through the stock and Termi they look exactly the same internally.
I think that Euro4 compliance has resulted in all the restriction happening before the slip-on so unless you go the racing Termi or a cat-delete, there will be little change.
I do love the look though and I would have put one on anyway (only with a heavy discount, not RRP).
The above being said, I was out all day yesterday on a loan 950 with stock exhaust (doing an off-road DARE ride on a hire 950) and when I got back on mine I though it sounded better when the valve opened. I'll need to do some more comparisons when I can.
I actually have a spare BNIB 950 Termi for sale (search on Gumtree Australia). I wanted the spoked wheels which were cheaper to buy in the Enduro Lite pack which came with a Termi.