Class 3 speed pedelecs allow up to 1 HP (750W) of motor assist, at up to 28 mph before cutting off. With a 750W motor, these easily beat roadies. The 250W ones should rival a roadie if a relatively fit person is riding it at a similar intensity level. Road pros might pull away on the hillclimbs, due to having a better W/kg ratio. Range is a factor too.
If it's not a class 1, then the "e-mtb" is something in between a mtb and moto, in a grey undefined area, that is generally not regarded to be a mtb or e-mtb. See the thread about non-class 1 ebikes for examples. Removing the 20 mph limiter immediately disqualifies it as being a class 1 e-mtb, which tends to be the only class of ebike allowed on bike trails, if allowed at all.
I do pretty big road rides on my e1mtb and rarely average 14 mph or above*, and my area is mostly flat. When I commute on my 29er singlespeed, I often average 15-17 mph over 8 miles (~30 mins +/- 5 mins).
Edit: *disclaimer, regarding my big road rides on my e1mtb: I'm mostly exploring new routes, rather than taking planned routes that are optimized for minimal stops and wide bicycle friendly pavement, hence my lower average speed. It's also not calibrated to my wheel, and its power steps down before I hit the speed cutoff; anything above about 18.2 mph is mostly my own power and I tend to not push beyond that, besides on downslopes.