I don't think the extra 2.5mm is the cause of your thigh pain over the course of 1.8km. I'm kind of with the others who say that 175mm should be no issue for someone 6ft tall. Understood that it's leg length that should determine size but unless you have extra weird proportions, your leg length should be fine given your height. I'm 5-7" and run 172.5 on two bikes and 170 on another. I could feel a little difference when riding the 170 for the first time, but that only lasted a few minutes. Lastly, if you didn't have the bike in hand before you ordered the power meter, I would let the seller know he's an idiot for giving you the incorrect specs.