I'm not convinced. I always pronounce it as if the 'w' is pronounce like a 'v' in both cases. But I'm more concerned about how you pronounce a double 'i' at the end of a name. I always pronounce it as in the letters 'e' 'i' said in succession.
I've done a lot of searching this morning on this. I've always pronounced 'ii' as 'ee eye' myself, but there is no consensus on this. Some say 'ee' some 'ee eye' it's one of those things that there is no right or wrong, it varies between countries and people - "whatever sounds right to You" is also what I've read.
As for woronowii, it's named after a Russian botanist, so his original name isn't spelt with our letters anyway and has been translated as both Woronow and Voronov. It is also a Polish surname, where it may? be pronounced as 'V' for both 'W's, but that's Polish not Russian. I have seen a break down of the pronounciation as wo-on-ov-ee-eye. So it appears that there is no right nor wrong for both the beginning nor ending.
The only agreement seems to be regarding the last 'w' as 'v'.
So whatever sounds right to You, is acceptable.