You need to know a factual *issue* about FM4.
Despite the updates introduced a fully-released physics for the wheels - you can feel incredible amount of tyre-thread simulation once you endulge into the Simulation steering setting without assists - the controllers are still having a built-in buffer for turning.
Thus, despite of the steering mode you're into (Normal or Simulation), the controller will always have *invisible* traction thank to that steering buffer. Result is simple - with the steering buffer, controller players do not fell any handicap regarding actual tyre-thread/tyre-threshold and they can enter and exit any turn more efficiently.
And because tyre-threshold is the single most important variable in movement-physics - which determines both entry and exiting capabilities and influences traction-offset in both longitudinal and lateral movement - result is that when playing with controller you can blast in and out of the turns without having almost any handicap regarding tyre-threshold. And that is the problem of course, because at the end not only that controller players can have much more stability and traction from the turns, they can also have a greater exit-speed because of the non-existence of the full thread-simulation.
Of course, another variable is 900-degree steering. Both Normal and Simulation modes have the same input-variables for the 900-steering, only difference being is the moment where tyre-thread is reaching its grip-peak (Simulation have a full-realistic simulation of peak, while Normal has it dumbed-down). What it does for your driving is allowing you to naturally steer the vehicle and apply the proper steer and countersteer. However, once controller comes into picture things again changes, because with the wheel have to cope with more than 3X more fidelity in steering-angle (controller have only 270 degree lateral angle), resulting with need to counter-act that fidelity with much more finesse and actual work with the wheel.
Controller players only have to "act" with small movement left-right (steering buffer does the rest for them) and they're out of the apex, whoosh - while you're still fighting with the countersteer.
However, please do not allow that reality of not be able to be 100% competitive against controller players disappoints you. Forza 4 has a stunning physics of tyres, probably best on any console ever, any only real way to experience that physics is by playing with the proper 900 wheel on Simulation setting. Luckily, you have the best wheel imaginable for that, CSRE does a magnificent job in taking advantage of the full physics-simulation of Forza 4 and transmitting them to the player.
The *issue* is 100% at software side, where Turn10 decided to somewhat "sacrifice" reality in favour of playability with decisions to leave the both buffers (longitudinal and lateral) working for controllers and not filtering the steering physics (Normal and Simulation) in any competitive mode of the game.
My personal advice is to just forget all of that and focus on your subjective and personal experience of the game. I have successfully overcome that complete situation in my mind and learned how to enjoy the game for myself. For playing both Career and Rivals modes I use Simulation physics exclusively and having the greatest time in any Forza game ever.
Once I go into online, I just switch to Normal steering (those options are found in the Difficulty menu prior to the race) and usually fight for the mid-tier positions with the rest of the pack.
Be patient and learn to love the sensation, not the performance. Once you overcome the reality of not being able to be equally competitive - but with cognition how your sensation is much better and much immersive than any controller can ever provide - you will start to appreciate both game and CSRE beyond any level imaginable. Regards!