Vulcan, like Earth, had its aggressive colonising period. Savage, even by Earth standards.
So, twice we get to hear the Vulcan colonization period directly compared with its Earth counterpart. Which, as far as we can tell, never involved space travel.
Yes, in the TOS context, it's possible that Earth had a savage period of brutal interstellar colonization, with fierce infighting, enslaving of little green men and so forth. In the wider Trek context, however, Spock must be interpreted as referring to what happened on Earth before
WWIII, first contact and warp flight. Is Spock comparing apples to oranges, then? Or is he saying that Vulcans, too, were savages before
going interstellar, and the only divergence between the two scenarios is that there is a slim but now apparently confirmed chance that some barbarians slipped into space at the moment of transition from savagery to civilization?
why did the exodus Vulcans embark voluntarily on a difficult journey (presumably nuBSG style?) to the farthest reaches of space ?
The obvious alternative is that Vulcan already held an interstellar empire at the time, and the various factions had offworld power bases; any faction defeated on Vulcan could fall back on its interstellar resources and sulk on some distant planet, plotting for revenge. If we assume this early interstellar empire had fairly primitive technology rather than replicators, we can then easily also postulate that the colonies on which Romulans were biding their time were incapable of supporting major industries or research, resulting in the degeneration of the Romulan culture in the intervening two millennia even though the original interstellar tech had been quite up to specs and had provided easy access between Romulus and Vulcan.
The "farthest reaches of space" thing has never been a major part of the Romulan identity in any case. Our heroes always easily reach the Romulan territory, even if they have some trouble communicating with home base; it's probably more an issue of bad relay networks than distance. Correspondingly, Romulans easily reach the heart of the Federation (significantly, Vulcan) simply by stepping out of the RNZ. It's not a distance easily spanned by sublight spacecraft, apparently - but it is one easily spanned at low warp speeds and with outdated vessels.
why did the exodus Vulcans voluntarily deprive themselves of warp drive technology so that a few thousand years later all they have is "simple impulse drive"
This has always seemed to me like a huge leap of illogic. Nowhere in "Balance of Terror" does any character suggest that Romulans as a culture would lack warp drive, even if the single vessel encountered is (mistakenly?) deemed to lack it. Encountering a modern tank today does not lead you to speculate that its operator does not know how to make flying machines!
It is also blatant in the episode that Romulan weapons travel at high warp, so any character speculating on the lack of Romulan warp technology would be wildly off base.
Perhaps the writer did intend to establish that the Romulans were up the interstellar creek without the warp paddle, but he failed miserably in that. It's very difficult to think that he would have been intending such a thing, though, as the danger of a Romulan interstellar break-out is the driving force of the adventure. If the Romulans just take potshots at ancient Earth fortresses, without the means of taking the battle farther out, then the problem could be solved by simply abandoning those fortresses!