Looking for informed opinions re the above for running after 4 and 8 ohm heads. Intending for studio use mainly but possible live applications if I become confident with it. Do these puppies actually do what they say on the box?

The Rivera Mini RockRec seems to set a really nice benchmark for performance vs value. Is there anything else I should be considering?

Charvel / Godin / Peavey / Marshall / AMT
The only one I've spent any real amount of time with was a Palmer PGA04. Obviously I can't speak for every loadbox/speaker sim, but it sounded great and did exactly what it was supposed to, so it's at least possible. 

I keep hearing good things about the Rivera units so I imagine those are also worth considering. The Two-Note lineup would be where I'd start my search if I were in the market for such a product again. 
Two Notes Torpedo Studio. At this point, probably the best on the market. 

The Palmer is traditional, and a pro's choice for years. 
I don't think I'd personally go with the Rivera (or the Marshall box, either). 
The new rock crusher either the we looks nice....

But you are already loading down the amp and emulating the cab. Don't get cute with it. Let digital do its thing and just get the two notes
Jumping on dat gear sig train.
PRS Hollowbody II / BKP Warpigs
Strandberg OS6T / BKP Aftermath
Strandberg OS7 / Lace Poopsticks
Skervesen Raptor 7FF / BKP Warpigs
Skervesen Raptor 6 NTB / BKP Juggernauts
Hapas Sludge 7 FF / Hapas Leviathan
Anderson Baritom / Motorcity Nuke BKP Sinner Anderson H2+
Warmoth Baritone / BKP Piledriver
Ibanez Rg2120x / BKP Nailbomb

Blackstar ID:Core Beam