Hi Nigel.....welcome to the Forum
If you give them your XKR and £7.5k you will end up with 650bhp....sounds reasonable to me.
Not sure what the mods are or indeed what the quality of workmanship is like so I would probably refer you to Google to carry out some research on the company, customer reviews, etc.
Let us know if you go this route as would love to see how it looks (and goes!)
Cheers, Trevor