Good idea, so I checked on heavy bullets in .311 to see what was available at MidwayUSA. The heaviest are Woodleigh 215 grainers, but they're hella-expensive. About $0.54 each. The next heaviest are 180 grainers from Speer, Sierra, and Remington. The Remingtons come to only $0.12 each.
I'm sure you could push either one to 1000 FPS pretty easily from the 7.62x39 since a 150 grain has no trouble getting to 2000 fps, but factory data seems hard to find.
In .308 however you can find inexpensive 220 grain bullets and a 240 grain Sierra Match King. So, if you have the means, the Whisper would be a better choice just based on bullet selection.