Most compacts true speed is around 15 mph even without an implement. I drive a Fastrac occasionally and its a flyer at around 38 mph my mates got a Ford Big 6 conversion and even with a 5 bladed plough mounted he will leave a modern JD 6430 standing on the road.
Coley the trouble with a lot of Japanese compacts like mine ( Yanmar F17D ) is the bottom link arms are so long, they are mounted just behind the axle, theres not a lot of swing but fast speeds take their toll on the ball joints.
I would love to have seen the trailer wobble, reminds me of an old 135 I had, used to pull a 5 ton grain trailer and anything above 10 mph was a nightmare, our Fenland roads are not the most tractor friendly, anyway this thing had more ripples than Fatima the belly dancer. I was poodling along one day and the drawbar snapped, I went one way and the trailer and grain ended up in 12 feet of water in a dyke. At least the ducks and fish had a good feed!