Cable Lengh

I was wondering if anyone had issue with length of the PSU cables reaching the motherboard headers in a Fractal Design - Define 7 XL, specifically the mobo 24-pin and cpu 4-pin. PSU: Corsair RM750X.

As per my previous topic, I'll be ordering custom cables, however I have to specify the lengths, and I am not sure which way to go on this. I'd rather extra length than not enough, but am I supposed to measure up my case? - Ideally there'd be some extra length if I ever decide to upgrade. Is it...

I'm using a Sabertooth Z97 and an 840 Evo. My sequential reads are around 350 and so are my writes. After reading a bit on the possible issues, I thought I might be using the wrong SATA ports...