I think a better idea is upgrading 1.4 and 1.6 petrol engines gearboxes, changing 5-th gear pinions. 5-th gear indead is too close to 4-th, on the motorways is not good at all, it revs the engine too much...
After some researches I found that the 1.4 diesel gearbox is the same, differs only 5-th gears pinions, that have a lower ratio, and gear box differential. So, the solution I think is to change 5-th gear pinions, but until I don't find a damaged diesel engine gearbox to recover 5-th gear pinions I'm not 100% sure. After this modiffication, allmost 10 mph can be earn in 5-th gear.