As BillJ said, Spock wasn't always the stoic, coldly logical character early in TOS, this Spock HASN'T led the same life as Spock Prime, and we never saw Spock under the emotional pressures he is under in ST09 in TOS. . .so really, you can't say how TOS Spock would react. . .
Considering how utterly gutted he looked when he thought he'd killed Kirk I think snapping when his whole planet had blown up was expected.
The most telling scene is after the mind meld on Delta Vega. Kirk had tears down his face, and is trying hard to pull himself together. Spock Prime apologizes, saying "emotional transferrence is an effect of the mind meld."
Spock is crying his eyes out over the deaths of the Romulans and the Vulcans, but he's not showing it (although his voice did get thick and teary when he said "Jim, I just lost my planet. I am emotionally compromised.")
Makes you wonder what was going on inside during some TOS episodes...