I had similar issues with top end speed on my RMartin that uses an internal 3 speed Shimano Nexus hub. I could pedal fairly fast once the motor cut out (~18ish mph), but I could only get about 20ish mph before the cadence made me look goofy.
Mine came with 18 tooth cog and I swapped it to a 16 tooth one (I ordered an 18 and 16 from JensonUSA) . I have plenty of hills, but most are not too bad, so I don’t notice the low end difference as much but I spend more time in second gear now instead of third, and my top end non goofy looking speed increased a couple miles a hour. I feel that my trips are about 1-2 mph faster with it.
Once the battery weakens up enough to where can’t make my daily commute, I’ll be looking at a kalkhoff. Expensive, but it should pay for itself in a year or so.