I think the x5460 is a pretty solid cpu anyway. The only major differences seem to be TDP is higher (not neccessarily a good thing), higher bus speeds and slightly higher frequency. If you buy an x5492 on aliexpress in £72 whereas the x5460 is only £19. So the x5460 is way more worth it in my opinion.
30-40%, sorry that's not going to happen, if there are more in the family then maybe 5-10% is what you are looking at. To get 30-40% you need to looking at least 4th gen i5, or maybe 3rd gen i7
I think the x5460 is a pretty solid cpu anyway. The only major differences seem to be TDP is higher (not neccessarily a good thing), higher bus speeds and slightly higher frequency. If you buy an x5492 on aliexpress in £72 whereas the x5460 is only £19. So the x5460 is way more worth it in my opinion.