Wouldn't allowing the automation to do its thing be the best in this situation? If you were in a conventional airplane, and your Vapp turned out to be Vref+20, would you fly a slower speed just for spacing?
The answer is OFFCOURSE NOT. Unable is the message to ATC. ATC may have it's requirements but in turbulence pilot has his own which have to override ATC. If you fly select speed or fly conventional aircraft then half wind+full gust is the lowest limit you can fly at. VFE and Vapp is the band ATC can only ask you fly within.