[The Squatter of Amon Rûdh]

Or if he had all 20 rings, and a fur coat he could be Liberace.

Or a cockney spiv:

(Sauron opens coat, displaying large array of rings) Can I interest any of you gentlemen in a Ring? Enhance your power; become great in the estimation of the world and impress the ladies. Once-in-a-lifetime offer and all for the trifling sum of your eternal subservience to my will. Did I say eternal subservience? I meant twelve silver pieces.

...if he was able to get his own back plus the other 19 or 15 (minus the ones consumed by dragon's) I think he would be more powerful than with just the one ring.

Probably so. Of course the rest of the Seven would have been a useful bargaining tool with the Dwarves, but with the One on his finger and Gondor crushed, I don't expect he'd need to bargain with anyone, so it's reasonable to assume that he wanted the Dwarf-rings for their own sake. What really would have set the seal on his authority would have been the acquisition of the Three.
Come to think of it, although they'd enhance his power, I'm not sure that Sauron would have got the full benefit of the Elven-rings (cf my earlier comments about personal characteristics).

This is by-the-by, but it occurred to me when I was thinking about Sauron bargaining with the Dwarves. I wonder if he'd been touting that Moria offer around all the great Dwarf-lords. It would be just like him to offer that to everyone, to try and stir up some sort of internecine dispute.