It's not really a case of speeds if it is good working order. I would imagine both the 2.25 Diesel and 2.5 N/A Diesel to give similar top speeds, due to the rev range and gearing. However, if you're having a hard time getting above 50mph on the flat then something will be seriously wrong (unless you're hauling a really heavy load).
Both a good 2.25 Diesel and 2.5 N/A Diesel should get a Series up to around 60mph (maybe 70 if you're lucky) flat out with an overdrive.