I am considering many of the same bindings- Flux 45s, Cartels, and Union Atlas bindings. Using with stiff DC Status boots and a few boards that are all on stiff side (A Frame, Berzerker, and Cobra). Mostly freeriding- no more park for me.
I want a responsive binding for most of the time- but one that is still fine for days of slower riding with my kids. Been riding for 20 years- old.
Per comment above- I have not ridden with Burton Re-flex. Does it really take away from the responsiveness?
I like the Burton toe strap over the Unions- based on store visit today.
I need to do more research on the 45s, but would appreciate any feedback.
Neither the Berzeker nor the Cobra would be considered 'on the stiff side' - they are both pretty much middle-of the-road flex.