I think it was just bit harder for a Rear Pump to pull or suck the fuel all that way.
Years ago I recall we had big issues with vapor lock on the 74 models during the hot weather. It likely was the same with the 73.5 CIS models too.
This continued over time for only those early CIS models. Never an issue with the later models as Porsche moved the pump up front from 76 on.
Len