As said above, cutting the chamber deeper is the worst thing you can do for flow. You can get plenty more volume in that chamber by leaning the walls back, I've managed about 26cc in the past on a turbo 940 casting without touching the beak area or the chamber roof. If the guides are shot you need to change them prior to cutting the seats, but if you want to measure the chamber volume at this stage either do a quick seat cut to clean things up or use a bit of grease on the valve to seal the gaps.
The best way to avoid any damage to the existing seats when grinding out the chamber is to pop some scrap valves in (just loosely, no springs needed), possibly a bit late for that now but I thought I would mention it!
The best way to avoid any damage to the existing seats when grinding out the chamber is to pop some scrap valves in (just loosely, no springs needed), possibly a bit late for that now but I thought I would mention it!