Just to add like any modification you should tell your insurance company, as you are in theory voiding your insurance if you don’t. Lots of insurance companies won’t allow any modifications.
I should have made it clearer, with a handset you have the original map saved so you can return it to stock yourself. These companies can also send you a single file and software so you can load it to your car with a laptop.
look up superchips . Collins performance, AMD. MRC in Banbury.
Dreamscience in Hull. Devil developments on south coast.
All of them have good reputations.
Likewise my car is as it left the factory.