There is literally no comparison. There's a difference of around 500 fps, with 7.62x54r being the slower of the two. Think about it this way, .45ACP is about 1000 fps and .44 Magnum can get up to 1400 or 1500 fps.
If your reloads get a 500 fps increase from factory speeds, without dropping down to a smaller bullet, that's an extreme overpressure. That'd be 7.62x54r +P+++. Look at a reloading guide, I have Lee and Vihtavuori guides. 7.62x54r will not get beyond 3000 fps unless you keep the bullets at or smaller than 150gr, and that's the high end. Shooting "overpressure" reloaded ammo out of a 70+ year old gun is a sure fire way to cause a catastrophic failure.
And yes, I'm planning on buying a Vepr.