Well, Nimoy's looping was according to editor Todd Ramsay, something that he & Wise were very impressed by (this was either in MAKING OF ST TMP and/or Sackett's STARLOG column.)
So that would suggest the mostly-cold timber is what they wanted.
I always thought the 'hey that's the REAL TOS Spock' sound of his voice after the second plasma energy bolt vanishes without killing them all was a bit out of character, because that is how he should start sounding much later, after the meld.
Then again, it could be hinting that the old Spock is just beneath the surface all along and he slips up a little when he replies to Kirk about it being a "logical assumption." (it got a big laugh opening day, presumably because everybody else thought it was the old Spock coming through there.)
The more I think about it, the more the latter notion has some weight to it, because I remember even in the novelization Spock catches himself enjoying exchanges with the humans ... then again, I'm probably overanalyzing the hell out of this!