VFD is the way to go.
Our whirlpool pump is also our kettle pump, vorlauf, & lauter pump. Would not think of using a pump without a VFD for any of it. It does make a difference, not so much in the velocity of the whirlpool, but because an unrestrained high-speed centrifugal pump is so rough on wort. Even with a slow whirlpool speed--maybe especially so because you're slowing down the flow to get a slow whirlpool, thrashing the crap out of the wort with a beater-shaped impeller while throttling through a butterfly valve. At least for a typical small brewery kit. What kind of floc is going to make it through that? Using a VFD will allow you the best of all options as you can gently push the wort at the ideal pump speed at any stage of the brewing process. I highly recommend them. Going to install one on my DE filter soon. Good luck & be gentle with your worts!
Phillip Kelm--Palau Brewing Company Manager
and Worldwide Brewery Installations