Hi Pete, I don't know if there is a linear relationship between spool up time and probability of flame out, but they are related for sure.
Since I increased the spool up time of my engines, I have never had a flame out again.
I think that spool up time is more a marketing issue than a necessity. Just need proper throttle management.
I have also found very useful to have a throttle curve that gives you about 40% of throttle at 25% stick movement and 65% at 50% stick movement. This set up makes the throttle response more linear, and gives a faster response "feeling" during taxi and approach.

Jack