without ladders just the rack itself maybe .5 to 1 mpg loss. when im loaded with tools and materials and ladders. i will expected 3-5 mpg loss for sure. easily 500-800 lbs.
i preferred the mega clamps no drillings. i rather pay the extra. rack wind deflector is necessary to mute any wind noise at any speed. its like $70 with tax from amazon. small price to pay for no wind noise.
that's a lot of weight after the rear bumper. i dont think thats ford's idea loading payload. this will help too.
https://timbren.com/products/frbsmv-ford-maverick-2022-2023-rear-kit#!year%3D2023%7C%7Cmake%3DFORD%7C%7Cmodel%3DMAVERICK%7C%7Csubmodel%3D2WD
i would rather go with this.
https://gofastcampers.com/blogs/manufacturing-updates/you-can-buy-a-gfc-platform-camper-and-ford-maverick-for-under-29-000
https://www.bulletpointmountingsolutions.com/products/rubigrid-2022-ford-maverick-platform-dash-mount-device-phone-holder
it is adhesive and 4 bolts. its very secured.