Rate and Angle
Best RoC occurs at the speed where greatest excess thrust (thrust available/thrust required graph plots) exists. You would be correct in selecting a slower airspeed to result in a higher ANGLE of climb, however the range of usable airspeeds (between where excess thrust=0 at the low and high velocity values) at altitude is relatively small. Vy speed is about in the middle of the range, and remains relatively constant, 275 transitioning to .76 as your Captain mentioned.
Slowing beyond Vy will only increase climb rate temporarily, but stabilizes to both a slower airspeed and climb rate. Given the very small ratio of climb rate reduction to reduced groundspeed, I would guess the resulting angle increase is negligible (at altitude).
Last edited by Armchair Astronaut; 15th Jun 2015 at 06:17.