I believe the も basically just acts as emphasis. This is supported by weblio's definition of ても (連語): 「て」を強める意を表す。
You could think of 考えてごらん as "You should think about it.". It's a straightforward suggestion.
By contrast, 考えてもごらん is "You should really, totally think about it, trust me, just do it!". ...But actually, I guess that only parallels one sort of case. In another case, like (from the sample sentences)...
Just think of his being a cabinet minister! / The idea of his being a minister!
it's more like "Just think about it, even for a second! (and you'll realize how crazy it is)". In any case, some sort of emphasis, like those.
In the sample sentences, that emphasis mostly seems to be accomplished with "Just (think about it)" (which is my first thought, too). So to put it simply, I'd consider it like the difference between, "I mean, think about it. There's no way it's real." and "I mean, just think about it! There's no way it's real." The latter kind of makes it sound a bit more like it should be dead obvious.