Does anyone here have any experience with one of these?

It would be awfully close to the old 358 NORMA Magnum.

Loading data is available in some of the older reloading manuals.

The Handloaders Manual shows the 358 with a 250gr bullet and 70gr of IMR4350 good for 2650fps; as well as referencing the 358 B-J Express with a 250gr bullet and 78gr of 4350 for 2900fps. That is a load from Ackley's books Volume 1. Dimensions make the B-J look like an "improved" version.

I would suggest using 35 Whelen data for a safe start.

If I was aware of the critter 30 yrs ago it would've been the route I took. Nice combo with the 425 Express! Killed this bull yesterday with the Norma.