The default T-150 wheel profile has a large 'deadzone' around the centre. I put 'deadzone' in quotation marks because you can still turn the car, but for the first 10°/-10° of rotation there's hardly any FFB resistance and then it ramps up suddenly. As you change direction in-game it feels like your banging into something. I tried to dial it out via the settings but nothing worked so I loading a different random profile, the Fanatec Clubsport v2.5. I had to un-invert the FFB but now my T-150 feels great - no centre 'deadzone' or 'notch'. Hope this helps anyone else having trouble.
Think I've just found the problem - the Force Feedback Minimum Force thing. Reduce it to zero and the problem goes away.