"The average speed can vary according to weather conditions and ability, but expect an overall average of around 17mph -19mph for a ride of 60 miles"
Furthermore, an average speed will have significantly faster and slower portions along the ride. The OCC will drop to far slower speeds uphill and will ride far faster on the downhill.
Yes, of course downhill is faster. On a mountain bike I've only knowingly once managed to break 30mph, but that was going for it and definitely not erring on being safe. My average road speed at the time was about 15mph.
The top speed for powered cycles should be around the normal average as then there's not a large differential between powered and unpowered bikes making it safer when they share road space.
That appears to be a ride where they've deliberately set a slower pace for a ride based around socializing. 12-13 mph is about right if you're going to be leisurely carrying on conversations with fellow riders. Any faster and wind noise becomes an issue. I can't talk to my fellow riders at 20+mph.
"lycra clad racing-bike riders" ride far faster.
Capping a vehicle at 15mph means you'll be constantly getting passed on trails. I have ridden a 15mph capped scooter, I was one of the slowest vehicles in my local area that day. I returned it.
The Oxford Cycling Club, pictured here http://www.oxfordcyclingclub.co.uk/the-kings-men/ claim an average of 20 km/h (13mph) and they're lycra clad racing-bike riders.
I think you can break 25km/h readily, but I don't think that means we should increase the speed allowed with power assist.
There are moves to make city traffic 32km/h (20mph) limited which would put paid to your 40km/h anyway.