when heads are flowed on a flow bench, they're allways bolted down to a bore plate simulating the cylinder sidewall, and yes, it will make a difference in flow, as the valve gets closer to the cylinder wall, it will be shrouded more, and recorded flow will drop. IIRC Ryan J did a flow test of a set of heads on like a 4.25" bore plate and the same heads on a 4" bore plate, and he saw over a 10% decrease in flow between the tests, IIRC.