the 950 should be able to get to 4.0 with little effort or even voltage increase. i run my 920 at 3.6 on stock volts.
voltage only makes an oc stable it doesnt allow the cpu to auto overclock/turbo because its higher. you need to set it up manually by using the multiplyer and blclk timer. as for holding back the gpu, thats unlikely. the 590 is powerful but not so much it will cause a first gen i7 to crumple.
it may be a bit limited with some game engines but thats not the hardware its bad programming and no amount of overclocking will put that rite.