I thought HR and power were fairly closely related and was one of the reference points for determining your fitness?
It obviously varies between individuals but if you maintain 200w at 140bpm and then 6 months later maintain 200w at 135bpm is this not a fair indication of improvement?
Also as the ride progresses the HR/Power becomes uncoupled and HR climbs for the same power. The longer this is delayed must be another indicator of improved fitness/stamina?