I think truly bright DC can get through GCSEs with minimal revision.
That said, many of those who get 9s will be doing revision now too, and it teaches good study habits.
At A-level, it depends on the subject, but for the one I teach (a science) they need a level of familiarity with mark schemes which are very picky, as a minimum.
The students achieving the best grades in my subject tell me they do at least 2 hours a week + homework, plus more leading up to tests. They wouldn't include things like organising notes in this. Some of these students are exceptionally clever and grasp complicated concepts very quickly- they're also clever enough to realise they won't achieve without putting some effort in.
The very bright students who don't revise much tend to get Bs (alongside those who are not as bright but work very hard).
Equally, you get the students who are reasonably able, coasted through GCSEs with a little bit of revision and intend to "cram at the end" for A-levels- they often come out with Cs/Ds.