There were a bunch of magazine articles... IIRC most at least mentioning efficiency improvements... and lower power requirements for the same performance of a non-pod installation. But I'd guess they got most of their spiel from the boat and engine/pod manufacturers.
OTOH, when they do the on-water tests, I think they now mostly use read-outs from the electronic engine displays... so if they included speeds/GPH/NMPG in their tables... I'd suspect they were in the ballpark of accurate. There may have been a few "comparison" reviews where a magazine tested essentially the same boat with a pod and a non-pod set-up. Otherwise, might have to find a way to compare those numbers with a non-pod installation in the same boat...
All this from memory, mind you, and mine isn't what it once was.
And this was from casual reading, passing the time, not info I was actively seeking out at the time...
And everyone knows how useful magazine reviews are.
-Chris