The SA-8 does have good output capability for an 8" for sure... It's just that when you achieve your output with more cone area and less by stroke, you get higher efficiency and less distortion. The motor of a subwoofer is not perfectly linear - especially overhung subs like Sundown. As it moves, the strength of the motor changes and creates distortion. The Dayton has more linearity than the SA-8 to begin with, due to motor design, and it also has to move less to provide the same output due to the cone area increase, further increasing sound quality.
The SA-8 isn't bad for what it is, but it's not a driver that I personally would recommend.
In the case of the Dayton, you will get free shipping to you and the Parts Express return policy is awesome. If you don't like it, you are only out the return shipping. I returned two woofers to P-E last week, and will be returning two more this week. No restocking fee, unlike Fi who have a 30% restocking fee. (Ridiculous much? I will never be ordering from them again.)