After changing to a powerflex upper mount the amount of vibration I had through the steering column on idle was unbearable. If I only drove on track I may have kept it but to live with in London traffic was a form of torture.I'm not sure if this has to do with the mentioned mounts as mine is doing this as well and I'm still running the stock ones. I actually suspect it's the combination of Cup dampers with H&R springs that are responsible for it in my case.
I think the best mod I've had is switching from a factory Renault seat to a Recaro with Rentune lowered seat base. The Renault seat is more comfy but the feedback with the Recaro (when lowered) makes the car.
Apart from that, changing the audio unit and speakers is a must if you love music.