Don't you need to use the entire rev range in each gear,

how do you use e.g. 0rpm (or idle rpm) in top? How do you hit max rpm in top on speed restricted roads?

You can however, hit max rpm in lower gears whilst staying below the speed limit ...

No, I understand that (and do this), but what about the higher gears? Otherwise we may as well just thrash it 'safely' in 2nd. The problem, as you rightly say, is maxing it out it top gear means significantly breaking the speed limit and very well driving in a dangerous manner.

Rather an expensive exercise if getting fined or needing to go for a 'track day' or legally find an accommodating former airbase with its runway intact!

I do remember an episode of Fifth Gear when they were testing products that claimed to boost engine performance and efficiency, and the 1990s VW Corrado a 'flat spot' on its acceleration curve, but was mostly 'cured' and returned to near new performance by relatively cheap things, a decent fuel additive (and just running it normally for a week) being the most cost-effective.

Probably worth doing once or twice a year as well as the Italian tune-up for people who mostly do motoring in slow-speed, busy roads.