Don't ask me, but when the wheel is in sim mode it seems like the steering will only go halfway to lock or even less (judging from the replays). So, unless you already tried: Step away from the Sim setting. I kinda like the 'Amateur' mode, as it seems to iron out some of the straightline wobble and gives you full steering angle as well.
As for the sim mode 'emulating' a bigger wheel radius, that was just me guessing.. but why else would it simply not allow more than half (or less) lock? Sim mode may be great for grip driving, but it's no good for drifting.
With that wheel, you also HAVE to hold on to it on the straight, or the car will wobble and eventually slide.. even when going 50kph -.- It should be pretty easy to hold straight though, at least in am mode. If not, maybe your suspension settings are way off, or you're spinning the rear wheels?