Well, to comment on the topic at hand, I'd have that say the Freeza would still have lost, even if he had not used the Kienzan. By that point in the fight, he was exhausted and his Ki was dropping rapidly while Goku was still in good form, seemingly not even really winded.
Also, Freeza's mental state was just...bad. His pride was ripped apart, his composure destroyed, and he was extremely desperate. I think that after being faced with, not only a stronger opponent, but a Super sayia-jin, Freeza was just not the same person at all. His mind was nearly shattered.
Evidence of this can be seen when he arrives on Earth and almost instantly starts seeing images of Goku and having flashbacks. He might cover it up well, but when faced with another Super Sayia-jin, Trunks, the flashbacks come in full force and he's pretty much finished at that point. One of the reasons I love that episode soo much is that it portrays Freeza's character soo well and just has such a different feel to it from the rest of the series.
In closing, as much as I love Freeza(he's my favorite villian), I still say, his reign was over once Goku turned into a Super Sayia-jin. Hope you all enjoy that little rant.

Never underestimate the predictability of stupiditiy. - Bullet Tooth Toney, Snatch
Are you a murderer, cause I am? - Faith, Angel the Series