Originally Posted by Smellyglove
Am I correct in assuming since the narrower part of my system is at the end it will not matter as much?
First, no, that isn't correct.
Second, just upgrade the HERMS coil, it will make a difference. Having one 6mm fitting isn't going to kill your flow. It would be better if the whole system was 10mm, but a 10mm + 6mm fitting is still better than a 6mm coil + a 6mm fitting.