Once you release the code base into the wild, it’s at your clients mercy.
We recently had client, live posting from her phone and not realizing that images she was uploading were at 8MPs.
So if at launch your tool tells me my site is smaller, faster, whatever vs that site. The numbers going forward can/will change.
If they check it after 6 months and numbers are different, are they coming back to you with support tickets? I don’t know. Frame it however you think this might work but if the client is not technical this can put them off quick.