No I haven't quite It is definately the cause of the issue, but the issue seems to be that BI is revising the Max Frame Rate setting upwards, and this is causing the CPU usage to skyrocket.I may have resolved the issue...
There was a single camera with a different Frame Rate (30). In the (actual, not Blue Iris) Camera settings I changed this to 15 like the rest, and everything came back to normal.
I don't think that the issue was native to the frame rate itself, but rather, the fact that there was a difference in frame rate between one and the other cameras.
Reading the manual, it appears that the fact that BI automatically adjusts the frame rate is intended behaviour, but then what can i do to fix my problem?
Basically, I set all my cameras up as 15FPS - CPU hovers around the 27% mark
After some time, the CPU levels skyrocket again - and i find that the Bi automatically set this rate to different values for each camera. The only way i found to resolve the CPU issue is to set these back all to 15FPS but then the cycle repeats itself.