When you are talking about the air used to cycle the bolt, you're talking about relatively small amounts of air, as compared to the volume and pressure used to propel the ball. So bear in mind we are talking about relatively small changes here.
Now, if the amount of air vented with each cycle is more or less the same, then with the lower pressure, we are releasing less air per cycle, so the lower LPR pressure should be more efficient, but as stated above, we're talking relatively small differences.
On the flip side of that, the slower bolt cycle can effect how efficient the bolt is at dealing with air used to fire the ball, so while you may gain some efficiency on the bolt cycle, you may lose some efficiency on the shot. So in balance, you may see very little difference. Really depends how well the bolt speed is maintained at lower LPR pressures.