I had done the research as part of my buyers guide,
and now made a sticky as requested 
(Dragrad)
Both the 75/75 Tourer and MG ZT/ZT-T dont have any additional towing weight differences, so a saloon can tow the same as a tourer. It should be noted that the LWB/LIMO cars are lsited at the bottom after considerable searching.
HOWEVER the engines make a massive difference to the towing weight and such, so please folks, keep your selves legal and your cars undamaged.
It should be noted the towing weights are as follows (these include the trailer you are towing and load, and not the load seperate as many people discover!)
Please note the KERB WEIGHT and 85% KERB WEIGHT are typical of the cars accross the Rover 75 and MG ZT range, and that they can vary as much as 50 kg depending on your spec, so this is an approx guide based on Rover information from several owners manuals.
1800cc Petrol - NONE turbo
Manual
• Car kerb weight: 1390kg saloon / 1460kg tourer
• 85% kerb weight: 1181kg saloon / 1200kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1200kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1425kg saloon / 1496kg tourer
• 85% kerb weight: 1211kg saloon / 1270kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1400kg
• Towing Weight, Unbraked: 750kg
1800cc Petrol Turbo
Manual
• Car kerb weight: 1415kg saloon / 1475kg tourer
• 85% kerb weight: 1203kg saloon / 1254kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1450kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1450kg saloon / 1510kg tourer
• 85% kerb weight: 1232kg saloon / 1283kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1450kg
• Towing Weight, Unbraked: 750kg
2000cc Petrol - V6 (150bhp Rover 75 only)
Manual
• Car kerb weight: 1445kg saloon / 1535kg tourer
• 85% kerb weight: 1228kg saloon / 1304kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1400kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1480kg saloon / 1570kg tourer
• 85% kerb weight: 1258kg saloon / 1334kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1400kg
• Towing Weight, Unbraked: 750kg
2500cc Petrol - V6 (160bhp MG ZT AND MANUAL only)
Manual
• Car kerb weight: 1445kg saloon / 1535kg tourer
• 85% kerb weight: 1228kg saloon / 1304kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1450kg
• Towing Weight, Unbraked: 750kg
2500cc Petrol - V6 (177bhp)
Manual
• Car kerb weight: 1445kg saloon / 1535kg tourer
• 85% kerb weight: 1228kg saloon / 1304kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1480kg saloon / 1570kg tourer
• 85% kerb weight: 1258kg saloon / 1334kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
2000cc Petrol - V6 (190bhp MG ZT 190 and MANUAL only)
Manual
• Car kerb weight: 1445kg saloon / 1535kg tourer
• 85% kerb weight: 1228kg saloon / 1304kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
4600cc Petrol - V8 (260 bhp - rear wheel drive)
Manual
• Car kerb weight: 1720kg saloon / 1770kg tourer
• 85% kerb weight: 1462kg saloon / 105kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1680kg saloon / 1740kg tourer
• 85% kerb weight: 1428kg saloon / 1479kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
2000cc Diesel - 115 bhp and 131 bhp models
Manual
• Car kerb weight: 1485kg saloon / 1560kg tourer
• 85% kerb weight: 1262kg saloon / 1326kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
Automatic
• Car kerb weight: 1520kg saloon / 1590kg tourer
• 85% kerb weight: 1292kg saloon / 1351kg tourer
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
As a side note, I dare say towing with the 131 would enjoy better MPG than the 115, so the x-power remap would almost certainly pay for itself in no time.
Long wheel base / Limo models
2.5 V6 Automatic
• Car kerb weight: 1570kg
• 85% kerb weight: 1334kg
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
Diesel Manual
• Car kerb weight: 1560kg
• 85% kerb weight: 1326kg
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg
Diesel Automatic
• Car kerb weight: 1590kg
• 85% kerb weight: 1351kg
• Max Ball Weight: 100kg
• Towing Weight: 1600kg
• Towing Weight, Unbraked: 750kg