Here is the conversation on the rocks between true form Cooler and base Goku after their fight, this is just prior to Cooler transforming into his final form.
Cooler
"Hmph, you do as well as I expected."
"Enough to have defeated my brother."
"However, the real Hell begins now!"
"Once you witness this Hell, I will show you no mercy, no matter how load you scream!"
"Not until I've torn you to bits."
"Hmph, one time more. I am able to transform myself one time more than my brother."
Goku
"what?!"
Cooler
"You should consider this an honor, you will be the first to see my ultimate transformation, and the last!!"
Cooler starts transforming
Goku
"Incredible, his ki is even greater than Frieza's."
Cooler finishes transforming
Cooler
"So, shall we begin?!"
Cooler flat out implies Base Goku is exhibiting powers greater than Freeza, which would at least be in the range of Super Saiyan Goku as seen on planet Namek.
Now here the crazy part part, After that, Cooler transforms, and Goku uses what is widely accepted as a 20x Kaioken/Kamehameha against him, and Cooler flies straight through it, without even having a scratch on him.
Do the math, and you can see Cooler has an absurd power, and is dozens of times more powerful than Frieza.
. I also assumed Goku used a Kaio-ken x20 against Coola and put his power during that at least as high as Freeza's full power.
movie 5 base Goku (post zenkai) > Freeza's full power
Kaioken x 20 Goku >>>> Freeza's full power
essentially, it's like this
SSJ Goku (movie 5) >> final form Cooler >> Kaioken x 20 Goku (movie 5) >>>> Base Goku [post zenkai] (movie 5) >=< SSJ Goku (Namek) > 100% true form Freeza = true form Cooler >> Base Goku [pre zenkai] (movie 5)