Here's my two cents. The more displacement you have, the less you need a power valve. On smaller displacement engines, a power valve helps with low end grunt when closed and allows for more top end when it opens up. Since a big bore already has low end grunt (even when ported for top end), it doesn't need a power valve. A power valve also tends to even out the power delivery, whereas a non-power valve will typically have a stronger hit. If you like a distinct hit in power and have the skill to ride on it, I'd recommend a non-power valve for a big bore. Plus you won't have to bother with cleaning a power valve.