Don't know if you're still looking into this or not mate (I know it was a while ago), but I've just replaced mine with this one from the States;
http://www.ebay.com/itm/FITS-INFINITI-G37-G37S-G35-NISSAN-370Z-370-Z-6MT-CLUTCH-PEDAL-ASSIST-SPRING-/201111152023?pt=Motors_Car_Truck_Parts_Accessories&hash=item2ed328a597&vxp=mtr
Cost less than £20 delivered and arrived within 7 days of despatch.
First impressions; clutch feels a bit less assisted, less springy (no sh! t, right? ) and there's definitely a wee bit more feel at the biting point - to me, any improvement over stock, however small, is worth it. I should've tried it with the spring out for comparison's sake, but it was raining and I was already fed-up lying upside down and half in the car!