How about from an E6750 OC'd to 3.6 Ghz to a Q9650 OC'd to 3.8 GHz and then back to the E6750? Yeah the Q9650 was fast, don't get me wrong. It also bumped my windows score from a 5.6 to a 5.9. The problem is the performance difference is not there, I had ment to give my E6750 to my cousin and upgrade, but the upgrade was no where NEAR worth it. If the Q9650 was $350-$400 then maybe, but it was not even close. The biggest difference I saw was converting 12GBs of videos, for the fun of it, I tested before and after and found them very close, so close that if I had just gotten rid of 1 200MB video for the E6750, it would have been the same. This is because the programs I used did not fully use all 4 cores, and this being the biggest difference I had found in anything that I had done, including HD encoding, made me think that the time just was not right for quadcores yet. I will admit I do miss the almost instantaneous start up from button press to ready to go, but I can wait 10 seconds. Honestly, do not buy a quadcore unless you have below an E6300, and if you do then get the E8400 or the Q6600, they are pretty much the exact same thing performance wise.
The fact you used software that was not quad-core optimized is not the porcessors fault.
And Yes, If you are doing lots of encoding you can save that much time or more.
And if you are actually running lots of apps at once, the difference is even more impressive.