VNE has nothing to do with GS.
If you descend at a constant IAS, in ISA atmosphere, your TAS (equal to GS without wind) will gradually decrease.
Up or down 600ft is approximately 1% supplementary difference between IAS and TAS. 9000ft will be about 15%, so VNE at FL90 will be about 225 kt, and I don't know your cruising speed but a very low tailwind would be enough to get this GS.
Very nice airplane the Bonanza !