I've seen tests showing how they'll expand to their peak moving at speeds as low as 800fps.
The existing 9mm 124grain slug could be re-sized to .357 and used as-is. Moving at 850 from a snub 38, it'd be a dominant round for the caliber, definately the first thing I'd pick. In 357 cases and 1,200fps or so from an SP101 or whatever, they'd really rock. They could use the same projectile for each, as these have an unusually broad range of effective velocities, which is why they're more or less the only round I'd pick for a 9mm/40S&W/45ACP carbine today.
The Cor-Bon Pow'RBall is now shipping in 40S&W, 10mm and "400 Cor-Bon" (which duplicates 10mm ballistics using a 45ACP case necked to 40). Tests by ammolabs shows how they work great at 40S&W speed but show clear signs of coming unglued at 10mm/400 velocities, all the classic signs of the nosecone coming apart and the final projectile ending up back near it's original caliber minus 25% or so of it's weight. I'm assuming Cor-Bon is now working on a "higher-speed-compatible" version. The same thing will happen if you shoot the 40S&W version from a carbine. Traditional JHPs can also come unglued due to being driven past the speed they were designed for.
The E-FMJ on the other hand strongly resists such "overspeed problems" yet expands at low velocity. Damn fine round...but doesn't have as much peak expansion.