In case you haven't noticed, it's the REGULAR sneak attack damage that is very rare now.
Epic stalker ring, whisperchain and planar focus are all very good items that already grant insightful.
For regular you have to look to tharne's goggles, upgraded wretched twilight or crystal cove pirate hat. Which give you just true seeing, just ghostly, or just +15 to a skill, making them hard to warrant a slot.
Normal sneak attack +6 would be truely exceptional to see on a slot-worthy item.