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So based on Intel's current product line and prices, it seems like the majority of chips they get off the line work as 2.3GHz parts, and a much smaller percentage function at 2.6GHz at a voltage low enough to meet the specifications for the chip used in the Mini.
So instead of thinking of a 2.3GHz chip as somehow substandard or defective, you could think of the 2.6GHz chips as the ones that just barely work at the low specified voltage.
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While I dont disagree, I cant fully agree either. There is no difference other then clock multipliers on all of the Ivy Core i7 CPUs other then over all quality of production. But being that the i7-3770 runs at 3.4Ghz, its a safe bet any CPU slower then 3Ghz is just as good and of same quality as the other. So to say that intels 2.3, 2.6 and 2.7Ghz CPUs offered in Minis are better then the other isnt accurate at least now. Believe it or not they will actually lower clock speeds on CPUs and sell them at lower speeds just to meet market demands. Overclockers love this and often buy CPUs advertised lower then ramp them back up to what they can actually run at and save on cost.
The only production quality issue is that many of the i7 desktop CPUs made in Malay had cheap thermal compound under the heat spreader. However those made in the Costa Rica facility had much better compound and do run cooler. I know I got lucky and got a Costa for my system and its run cool ever overclocked to 4.2Ghz. Not sure were the mobile cpus were made for the minis, but I can say mine (i7-3720QM 2.6Ghz) runs very cool.
But to answer the OPs question is it worth it. I thought it was to me. 4 real cores at 2.3 -vs- 2.6. That 1200Mhz more power when not in turbo mode. One way of looking at it is thats a half a core more power. Since you cant upgrade it later on, it seemed logical to go with the 2.6. They did offer a 2.7Ghz model for 250 USD more. Now that IMHO was NOT worth the money. The chips themselves barely cost 250. I only gave 299.00 for my i7-3770 in my gaming rig. So IMHO 2.6 is the best value in the long run. Its only a hundred bucks, so if you keep the system for 2 years before upgrading. Then your only looking at $4.16 more per month for the investment. If $4 a month doent hurt your financially, then I would go for it.