So just how good are these SBM engines

Just how good are they? Well, like any other engine out there, they have there pro's and con's. 1 con is oiling. For serious performance useage, you'll need to drill the passages in the block larger. There's no real need for hi volume oil pumps.

A semi con is the cylinder head. While it has an advantage with 18* valves as stock and a easy upgradeable rocker system on a central bar, the cam at 59*'s puts the pushrods at a odd angle to the rockers. You will loose a little lift. Serious efforts in power will look to the race blocks 48* tapper angle. This becomes expensive.

The valve train of the LA engine will not need a girdle but the Magnums will since it is more Chevy like. Which also makes it cheaper.

The rest of the engine is excellent for what ever your looking to do for most of what could be done or sought.

The blocks are a high nickel content and can swallow up a 4.25 stroke. After market (MP) blocks can be bored out, up to 4.22. In theory, you could build a 472 cube small block.

Heads to feed it can get pricey though at this stage, who cares?!?!
That's my take!