I’m pretty used to the concessive conditional ーても (“even if…”) but I’ve read that ーたとしても can also be used.
The example given is:-
もし僕が金持ちでも、すぐには問題を解決できないだろう。
もし僕が金持ちだったとしても、すぐには問題を解決できないだろう。
Even if I were rich, I couldn’t solve the problem right away.
(Actually the book says “If I were rich, I could solve the problem right away” but that’s obviously an error - the point is that it’s saying both sentences have the same meaning.)
The problem I have is, the book stops there. It says only that ーたとしても can be used; it doesn’t say when or why you might use it, or what kind of nuance it has. So… anybody have any ideas?
Oh, I see. I was confused because ーても can also be translated by the subjunctive (as seen in the example), but I guess, to put it plainly, ーたとしても is always subjunctive and ーても is only sometimes subjunctive. So ーたとしても sort of emphasises the unreality (predictive, hypothetical or counterfactual) nature of the condition.
To add to this after consulting “All about particles, A handbook of Japanese function words”, here they state that one of the meanings of ～ても is ‘even if’ as OP has in the examples, it also states the usages:
‘no matter wh- o/ at/ ere etc.’
武田さんは、いくら飲んでも酔わないんですよ。
No matter how much Takeda drinks, he doesn’t get drunk.
‘at the most’ (emphasizing an upper limit)
そのカメラは、高くても５万円くらいでしょう。
At most, that camera will cost around ￥50,000.
(lit. That camera, even if it’s expensive*, will be around ￥50,000.)
Now the noteworthy point about the above two is that they, even if English doesn’t use a subjunctive in translating them, are irrealis to some degree; they don’t reference a real event, at least not entirely. So my two cents on the matter is that the difference might just be that ～ても has a broader usage, in that only in a phrase with the meaning ‘even if’ can ～たとしても be used.
PS. * Here I think that ‘even if it were expensive’ should work as well. However I’m not too versed in formal English grammar. In my own speech I tend not to use the subjunctive mood.