I have the 1.6 110BHP, and I tow about 850kgs. It is very happy at 60mph on the flat or gently undulating. We chugged up a couple of long non-motorway hills at 30mph in the French Ardennes this summer, but it was definitely hilly. Travelling from Delft to Rudesheim, towing at 50mph, we managed 48mpg - calculated properly, rather than from the computer. Beat that!
I have driven the 136 HDi in a 407 saloon, and it went like a piece of muck off a shovel. Should be good in the 307. Watch the noseweight though, as I think the limit is 60kg.