Hocchan's Saviours. (Baka Test)

[hobogunner]

Okay, so, now I'm slightly confused as to Akihisa's phrasing. Yuuji said some insanely smart thing beforehand (I sort of only translated half of it, I'll tackle it later.) And so, Akihisa is amazed, of course.

そう言って、雄二は歴史上の人物を数名挙げた。なるほど。そうなのか。

I get the "Saying that, Yuuji cited a few historical figures." however, I don't understand the なるほど。そうなのか。part. It's him going "I see. Do I really?" basically, so I don't know if I should simply put "I see....?" I don't think it's too much of a stretch. Maybe "I see. Really?"

I'm putting down "I see. Really?" for now.

[joay_b]

The "そうなのか。" part can be interpreted as a rhetorical question (i.e. a question whose answer is already decided by the speaker). So the speaker here is saying "is that it?" when he really meant "so that's it."

ただの意見です。

[larethian]

Okay, so, now I'm slightly confused as to Akihisa's phrasing. Yuuji said some insanely smart thing beforehand (I sort of only translated half of it, I'll tackle it later.) And so, Akihisa is amazed, of course.

そう言って、雄二は歴史上の人物を数名挙げた。なるほど。そうなのか。

I get the "Saying that, Yuuji cited a few historical figures." however, I don't understand the なるほど。そうなのか。part. It's him going "I see. Do I really?" basically, so I don't know if I should simply put "I see....?" I don't think it's too much of a stretch. Maybe "I see. Really?"

I'm putting down "I see. Really?" for now.

なるほど。そうなのか
I see. So that's how it is, huh?

Perhaps it is easier to see the usage of の in a more polite form, since it often appears as ので / んで, のだ / んだ, which has a few variations in usages (though they are more or less based on the same functionality of expressing reason).
そうなのか = そうなのですか

In a way, it is as what joay said. Here the use of の suggests that Akihisa is not 100% sure and is asking for confirmation from someone or no one in particular (a slight mulling/doubtful kind of thought), but if it ends with だ rather than か, then it suggests an answering to his earlier doubt (this will be exactly as what joay said).

In an exchange, when a person uses の / ん in this manner to ask a question (asking for confirmation or as a 'softened' query), the person who replies will usually use の / ん as well in his answer.
Eg.
Q: そうなんですか？
A: はい、そうなんです。(instead of just そうです)

Q: 帰るのか？
A: ええ、帰るのだ。 (instead of just 帰る)

So your translation is not 100% wrong, though I think just saying "really?" has way too much doubt in it.