Filling in the cavities is going to raise the compression and get you more power, in theory.
I try to use a mix of silly putty with a handful of aluminum powder mixed in. It gives me the proper ratio at lower rpm's but compresses at higher rpm's giving me a variable compression system. The best of both worlds without the mess and headache of exhaust valves. I'm toying around with an ether fogging system, it involves a roll of duct tape, 12v solenoids, body armor, good medical insurance and a guaranteed 2 million views on you tube.
Er...... Scratch that last idea.... The wife said no..........