A guy named A.Graham Bell once wrote a performance book and I think that's where I read about verified up to 10% increased flow, or was it even 10% increased power. Not sure anymore, don't even know where my book is these days.
And yes the exhaust valve front cut is on the chamber side of the valve, usually less is cut than in a typical back cut. I don't remember the theory behind it, but I suppose a sharp 90deg edge would cause harmful swirls in gas flow or whatever, and the front cut would eliminate some of it or create a more beneficial swirl pattern that helps expel the gases past the valve.
Edit:
http://www.goodreads.com/book/show/3613477-four-stroke-performance-tuning
