This is utter bunk! A 148 Gr. .38 WC and 240 Gr. .44 have nearly identical sectional density, or in other words they are close to the same length! Bullet length as it relates to twist rate is what stabilizes bullets. The .44 will be just as stable as the .38 at the same velocity. If you experience tumbling from a .44 full wad-cutter there is something else wrong, not the bullet shape!
I have been shooting cast bullets, and even designed several, so I think I have a bit of experience with this question! I guess the real question is why you would even want to drive a .44 WC over 950 FPS? Just because it is in a magnum case? Any bullet that works well in .44 Russian, and .44 Special will work just as well in a .44 Magnum case as long as everything else is correct.