The 0.659 5th gear mod I had done probably doesn't drop the revs enough at m-way speed to have had a dramatic MPG benefit. I've got used to the extended rev-drop but in both cases can see how it's a compromise solution and that a 6th gear would be better.
When motorway driving, the 0.659 5th gear increases my MPG significantly. Earlier in the summer I did a journey where I averaged 78mpg, which would have been impossible with a standard 5th given my lardy A2 with its 17" wheels and 205 tyres.
Whether the 0.659 5th reduces your overall consumption depends greatly on how much you end up using 4th. With a longer 5th, you can find yourself doing high revs in 4th quite frequently. As such, sometimes the longer 5th saves you money, and sometimes it costs you money. Whether the change of 5th gear makes financial sense depends greatly on how you use the car. Since I had the 0.659 5th put in, I've done thousands and thousands of motorway miles and I'm confident that it has, overall, saved me money (not taking the capital investment into account). However, there have been times when it's cost me a bit. I find longer 5th almost useless when driving in The Lakes, so weekends in the hills use a little more fuel than they did before the change of 5th.
As I have said before, adding a 6th gear is the ultimate solution; not only can 6th be longer still than the 0.659 5th, meaning even better motorway economy, but it also means having a standard 5th gear, meaning no high revs in 4th.
I've got used to running a longer 5th and, if limited to 5 gears, it remains my setup of choice. However, just like there are limitations to having a standard 5th, there are limitations to the longer 5th. It should be seen as a change, not as an upgrade. When I'm 'having fun' with the car, the magic ends when changing up from 4th to 5th. The overall 'driveability' of the machine is somewhat reduced. There are also certain cruising speeds and road conditions that leave you constantly 'managing the gap', which can get tiresome.
I drove an A2 TDI with a standard 5th recently and really enjoyed it, until I got on the motorway. The extra noise and vibration, even at 60mph, was quite wearing and I was once again reminded why I have my own gearing the way it is.
Tom