I think leaving the final drive / differential the same and just changing the 5th gear cog is the way to go.
It would leave the car completely unchanged and driving as it does now, until you get into top gear. Then instead of turning e.g. 4000rpm at 80mph, you'll be using ~3500 instead.
There will be a bigger gap between 4th and 5th, so it may mean you can't change into top quite so early... but I don't think this will be a huge issue with the fairly small gearing change being suggested.
The new cog won't put the engine waaaaay out of its torque band so it will still accelerate in top gear, just slighly
less quickly... but when at a steady speed in top, it will just be running with less rpm. That saves benzina, keeps the temperature down a bit in the summer, and makes the trip slightly less tiring on the crew.
The top end speed will probably be unaffected. The taller gear will give you a higher theoretical top speed but the engine's power won't allow the car to reach it. It'll end up pulling the maximum rpm it can reach with the power it has ... and I'm guessing it'll be coincidentally the same speed as it does now, give or take the odd mph... and albeit at lower rpm.
But the main rationale for the long 5th is to reduce the rpm when cruising. I guess an
answer may be to "buy a diesel"....
but I've got benzina because we likes it... and if there's a way to make it more user-friendly on the motorways, then it's worth investigating..