Thanks @Koinu for doing the initial testing, and @DeanMarkTaylor for the extra data.
It’s important to note that it’s 13.84MB on an 8,712.40MB archive – an increase of 0.15%. That really is down in the noise, IMO. The increase in time is more worrying: it’s somewhere in the vicinity of a 10% increase, which, while it’s a background process, is still a fair chunk more time. I’m honestly quite surprised at the increase in time taken, I didn’t think it had that much impact. Some hand-wavey tests on a local file here seem to support the time increase you saw.
I’m not a fan of adding a site setting for something like this. People will not turn it on when they should, or turn it on when they shouldn’t… I don’t think it’ll be a good outcome. Despite the increase in time, I’m inclined to turn it on, because it does have some significant benefits for anyone doing dedup (or any other delta-observant process, like, say, rsync) and shouldn’t have any observable impact on other people.