I designed the product inspection test skid for Fallbrook's e-bike IGH combo electronic cadence control unit years back. When we went to commission it in Austin, they had us ride e-bikes to lunch with their stuff on it.
The % assist you could ask for to me seemed to work really really well. Like, it wasn't like a 4-position selector switch. It really felt like it had a good working range of assist.
I would investigate why your bike is having trouble with the granularity of the assist level it is providing. That could really improve your user experience and data you're looking for.
But, for road bike, I'd say that wattage of over 200w on flat for 20mph would be probably on a pretty non-aero setup on standard tires. I ride a pretty aggressive setup road bike and I do rides that average 20mph with 60 feet per mile elevation on only about 205w or so AP. Then again, my Stages power meter is probably junk.