I doubt lean burn is worth more than maybe 5mpg on average, but it depends on some variables, such as how fast you're driving. Cruising along at 30-40 on flat roads you'd be at very low loads without lean burn, whereas at ~60 (close to the max speed you can get in lean burn) the gains are much smaller, since the throttle plate is still relatively open and load pretty high even at stoich. The Insight's engine runs very well at low RPM and I expect 130+ might be achievable with hyperinflated tires cruising along at ~30 in 5th using lean burn, and you wouldn't come close to that without it.
Pulse and glide in lean burn at ~50 in good weather:
Steady cruising in lean burn at ~50 in good weather:
The IMA system probably helps more in the city, where regenerative braking can recapture some energy, but mostly it has to do with drivability. The Insight has gobs of torque at low RPM when you have a healthy battery.