This we don't know as of yet.
We know that good technique can stop him from using it, but not if its easy to return. It's actually near impossible to return.
Kenya winning, I believe is what will happen though.
Still, although Kenya is now God's Speed with Perfect balance, it makes him an amazing 100m athlete like Eiji Kikumaru has referred him to Usain Bolt. It doesn't necessarily make him too much of a better tennis player.
Kenya should win, only because he is good enough to lose 7-3 to Shiraishi.
Jirou serve lets him get to the net real quick. The issue is does Kenya have the necessary technique to seal away his Volleying ability.
This we do not know.
I say Oshitari Kenya 6-3 win.
Lol, you are saying "this guy is right, but Kenya still wins because he won 3 points against Shiraishi". We don't know if Jirou is good enough to take 3 points from Shiraishi, but he probably is. The 5 of you are focusing too much on those 3 points. This is Jirou vs Kenya not Kenya vs Shiraishi and Jirou vs Fuji. I made up a scenario in which Jirou wins this match, can't you do the same?
Kenya can't do Hakugei, that's for sure.
Edit: those 3 points are probably Konomi randomly saying "I'll make Shiraishi win, but I don't want a total beatdown... Yeah, 7-3 is fine."
Edit 2: Now I'll discuss he same way you do. Jirou won without a problem against Kabaji, and Kabaji almost won against Nationals Tezuka... Do you see Kenya doing that?
I know it sounds like a stupid comparission, and that's exactly how the 3 points thing sounds to me.