That's the laws of physics, Swifty. The power demand to counter air drag is a cubic function. Expect 25 mph with the maximum assistance. 28 mph is only doable downhill, at least with mild slope. And, preferably, some tailwind.
I had interesting situations on my last vacation in the mountains (I was riding an e-bike equivalent to yours). While riding steep downhill, I didn't need to pedal, and reached 68.4 km/h (42.5 mph) on one day. On another day, I was riding downhill on a moderate slope; a roadie took me over. I discovered I needed to pedal to be able to catch up with him, so I switched to Turbo assistance. Guess what. I hit the 45 km/h (28 mph) limiter, and the guy was simply faster than I on his traditional road bike...