mate, you can still drive along in 3rd gear if you want but with the upper six gear you now can get the fuel-economy which just didn't exist before. There's no difference between 5th and 6th gear anyway, so my guess is you'd stick with 5th anyway.2500rpm at 80mph? Having to change down on the motorway to get an overtake done doesn’t sound like much fun.
Another recommendation for this from me as well, one of the first things I had to do was get the end bearings changed out, great service from A&S.There is a fella in Halifax, A&S Clutch technology who is very well recommended and changes all the gear cluster bearings in situ in less than a day for about £300 and all guaranteed.
His every day work is Vauxhalls but does Alfas with the same m32 box.
Where ever you are in the uk its worth considering, even allowing for the price of a tank of fuel and a night in a travelodge.