Some Europian bikes use the ten percent leeway in the regs to allow a bit over 17mph before they cut out. Kalkhoff do with all their bikes, but I’m not sure how many others do it. On mine the power progressively eases off until the cut off is unnoticeable and you are on your own. My Kalkhoff and other ones I‘ve ridden can be pedalled past the cut out on the flat well too, which not all ebikes can be. They are CD, but the main thing is that they have good components and tyres which not all do. That’s what makes pedalling unassisted like riding through deep water with some bikes. Weight also matters but that isn’t so noticeable when the bike is up and rolling.Thanks for suggestions. So would a hub motor be better than a crank drive motor if I wanted to take the bike beyond its 15 mph range. One issue I don't like with my current ebike is that when I start pedalling the power is delivered as one and takes the bike straight to 15mph if it is on a straight. I then struggle to take it beyond at best 17 mph no matter how hard I pedal
A cheaper Chinese e bike I had as my first ebike was a pig to pedal past the cut out. Which at 15.5mph was also like hitting a wall because it didn't ease down the power as the cut out speed approached. The only way to make that smooth was to change the cut out to a higher speed which also made it illegal.
On that point your OP was confusing in that you say you are averaging 17/18mph and want more speed but also say you want to stay legal. Your average is over the legal speed now and to average that you must be going well over 20mph at times. So you can’t do that assisted and be legal. But if you want a more efficient bike which will assist you on hills and the flat up to 17mph legally and is easier to ride past the cut out then a Kalkhoff is a good bet. That or a buy a good bike and convert it.