i'd go ahead and bite the bullet - the late 2015 27" imac's still have great specs that wouldn't be too much improved with a new release at this point anyway (unless they did a huge redesign which certainly won't be in march anyway) - i was in the same boat and just picked up the base model late 2015 27" yesterday and don't believe i'll be regretting it at all. not only does it not seem imacs will be ignored in this march update but also unsure there will be much of an event at all (slight phone and ipad updates? good grief).
I just ordered. I'm not what's considered a 'power user' anyways. I am looking forward to checking out that 5k screen. I haven't seen it in person yet.