Well, I took the rifle (Springfield Armory 'Standard' version) to my local range last Wednesday and today... set up two targets at 200 yds. and a chronograph. I fired three series of twenty rounds each (handloads using the Hornady 150 gr. BT-FMJ and CBC cases) over the chronograph...five with the spindle valve turned OFF, followed by five with the spindle valve turned on. I then fired five rounds at each of the 200 yd. targets to see if there was any effect on POI and grouping in either mode.
There was only a 6.4 fps difference in velocity between the two modes in the first series. The rifle shot 2679.2 fps with the valve OFF, and 2685.6 fps with the valve ON.
There was an interesting development. I ended my shooting on Monday after the first series of 20 rounds because I felt that the handload I was using was too hot. While my powder charge was within Hornady's published 7.62 X 51 data (sixth edition) for the bullet and powder I was using, and there were no outward signs of excessive pressure like flattened/cratered primers (there was in fact still some radius on them), it was very difficult to manually extract the fired cases. This wasn't noticeable while firing the rifle with the spindle valve ON. I went home and reduced the powder charge by 1.0 grain and returned to the range today.
I fired the remaining two series of 20 rounds in the same fashion as before. The second series used the CBC cases that I used before, and the third series used WCC-07 cases. I had no difficulty with manual extraction with any of the fired cases today. The second series (with the CBC cases) gave 2601 fps with the valve ON, and 2591.6 fps with the valve OFF (10.6 fps difference). The third series (with the WCC-07 cases) gave 2579.5 with the valve ON and 2579.8 (.3 fps difference) with the valve OFF.
There was absolutely no noticeable difference in group size or point of impact in either mode of operation in any of the (6) 200 yd. groups I shot.