This chart shows the two alternatives - the standard 0.65, and the 0.59 overdrive. He hasn't got the wheel circumference right so the speeds aren't accurate.
So at 80mph, a drop of either 400 or 600 rpm.
Riverlad's chart shows that the torque peak is at 2200 rpm.
With the 0.59 gear, at 60mph (1750 rpm) there is 50hp available, rising quickly up to 2200rpm (70), and then gently increasing after that. 50hp is fine for cruising along steadily (I think the car needs around 20hp for 60mph), or going up gentle hills, but it would probably need to change down to 5th for steeper hills or acceleration.
At 80mph (2400 or so) there is plenty of torque & hp available, but it would probably still be a good idea to change down to 5th for acceleration in order to reduce the strain on the clutch.
There is the potential cruise control issue - although personally, while I use cruise daily, it is only in a 50mph average speed camera zone, and I don't use it in normal driving.
With the 0.65 option, it is exactly the same as many of the 5th gears already on the forum, but with the original 0.75 still available (so a narrow gap between 5th and 6th).
The 0.62 is not available from EuroTuning.
Interestingly enough I was talking to a Ecomotive (Seat) driver yesterday & he said the standard gearing is around 1600-ish rpm for 60mph!! He was having problems with the DPF regeneration (not entirely surprising).