I saw a similar combo (using Wyvang not Genbull) in a LTD discussion thread tested against a Bakushin LTDC, something I'd meant to do. So I borrowed GuardianRevizer's Bakushin for a bit (he was whining about the paint getting scratched up so I only got 10 rounds in) and did some testing.
DK Genbull 85RF vs MF-H Bakushin Sagittario II 90RB
Genbull wins: 3 (2 OS, 1 KO)
Bakushin wins: 7 (6 OS, 1 KO)
Genbull %: 30%
Genbull really couldn't effectively spin-steal from Bakushin; Bakushin's sloped sides and low track made it difficult to properly spin-steal, and KOing only worked once when Genbull smashed HARD into Bakushin right at the start. My second attempt at that strategy resulted in Genbull deflecting off of Bakushin and earning Bakushin a KO instead.
EDIT: 900th post! yay arbitrary numbers
I also did some more testing, which turned out to be rather unusual.
First the benchmark th!nk asked for forever ago
MF Gravity Perseus 100RF (seemed to match DK Genbull's profile best at that height, though 90 might have been better, idk, too late now
) vs MF-H Libra TR145RB
Gravity wins: 15 (5 OS 10 KO)
Libra wins: 5 (4 OS 1 KO)
Gravity %: 75%
Interesting thing here is that I was attempting to replicate Genbull's use in Gravity, to make it a better benchmark - I played it safe and went for an OS rather than trying for a OHKO a lot of the time, using this combo more like an aggro spin-stealer than an Attack custom. That Gravity STILL got KOs half the time with an eerie degree of ease really shows how it deserved its ban... I could probably have hit 90% or so if I went for the KO straight out rather than going for a more Stamina-oriented approach.
Next thing is Genbull vs MF-H Libra 230CS.
Genbull wins: 8 (all KO)
Libra wins: 12 (all OS)
Genbull %: 40%
I was expecting complete domination by Libra here. When I did the first round and Libra went sailing out of the stadium, I was shocked. I thought Genbull was way too short to touch Libra. However, if you get Genbull to bank and hold an angle, it can reach up juuust enough to nail Libra's underside HARD. I was plagued by wall-saves and good hits at the wrong time here, so the results may be a tad skewed, though I doubt Genbull could pull off more than 60% even on a really good day. Still surprising, though.