Did it really cut into the valve seat itself, or just into the transition region? If it truly cut into the valve seat, then I'd think you're headed for trouble. If it just cut into the transition region, then probably nothing to worry about. I can't believe that taking 0.020" off the heads actually got it that close to the valve seats. Have they been milled before? The only real way to determing combustion chamber size is to have it measured. As shop should be able to do this for you very cheaply.
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Jeff Chambers
T/S #4
12.40 Seconds/109.5 MPH
http://www.kellnet.com/chambers/default.htm