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I have an answer to this. Most Apple stores carry a small number of "specced out" models in stock that live in a gray area between stock and BTO. They are a little hard to find but not impossible. The specced out "stock" 15" is a 3.1Ghz 1 terabyte model, which I purchased rather than a truly BTO 3.1 Ghz 512GB model for precisely for the reasons you suggested.
Yeah they are hard to find but only problem is I don’t wanna pay so much for 1tb as I use sad hard drives a lot, I wanna go today refund my 3.1 and get the 2.9 as I’ll save £190 and if there’s a problem it’s easy to swap, just for my heads up plz tell me I won’t see much diffrance between 2.9 and 3.1 as it’s same chip just slightly over clocked
 
Yeah they are hard to find but only problem is I don’t wanna pay so much for 1tb as I use sad hard drives a lot, I wanna go today refund my 3.1 and get the 2.9 as I’ll save £190 and if there’s a problem it’s easy to swap, just for my heads up plz tell me I won’t see much diffrance between 2.9 and 3.1 as it’s same chip just slightly over clocked

OK. My single core benchmarks are identical (not sure why -- 3.1 matches Geekbench, but my 2.9s were much higher). I only went up to 1TB because someone else was footing the bill. Stick with the 2.9 and be happy!
 
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