I remain a fan of the longer 5th setup, despite now having a 6-speed 'box. However, I'm adamant that it needs to be thought of as a change rather than as an upgrade. It's a compromise solution that provides some pros and some cons; whether or not it's suitable for you depends greatly on your use of the car.
I predominantly do motorway miles, so my previous longer 5th setup would frequently save me money due to the increase in fuel economy at motorway speeds. However, I also do a fair amount of driving in mountainous areas, where I would essentially only have 4 gears due to the longer 5th being inaccessible in such environments. In these circumstances, the longer 5th ratio would effectively cost me money. On the whole, I saved, but many A2 owners with the longer 5th find that their average fuel economy does not go up by much, if at all. Sometimes, all that's gained from the mod is a quieter drive when motorway cruising - which alone can be a good justification for the change.
Personally, I find the longer 5th is a down-grade in terms of the vehicle's 'driveability'. On a winding A-road (or in a 50mph speed limit on the motorway) I found managing the larger gap between 4th and 5th really tiresome. When doing longer cross-country routes, I found it quite draining to have to continually choose between rev'ing it harder in 4th or spluttering slightly in 5th; it requires much more concentration on an aspect of driving that's normally a subconscious process. Low-rev, high-load scenarios (such as trying to accelerate from 45mph in longer 5th) are terrible for your engine and gearbox, meaning the lower limits of the longer 5th's capabilities ought not be explored too often.
For those who use their A2 almost exclusively as a motorway cruiser, it's a very good option; compared to a 6-speed 'box, its pay-back time is relatively short and it makes driving at speed a whole lot more comfortable. But it's not without its drawbacks.
Cheers,
Tom