I use this junction every day.
I also suggested this, amongst other things, to the council a few mmonths ago. I'm extremely annoyed that they've implemented a fairly minor improvement whilst completely ignoring far more significant improvements such as pedestrian crossings on Kirk Brae and Mayfield Road.
Actual experience though: The head start itself is more helpful than I was expecting. I can now get from Mayfield Road to Kirk Brae before the "real" traffic catchrs up, which makes it far easier to take a strong position up the hill.
However, it's still really difficult to actually filter to the front, making it hard to take advantage of any improvement. Indeed, traffic seems to back up further on Mayfield Road than it did before, making it take more light cycles to get through the junction (turning onto Mayfield Road from KB is a particular problem). I can only hope tthis will improve as traffic displaces oon to other routes
All in all, not much use in this particula location without other improvements.