I've never used Litchfield but they've been around for years, upgrading and tuning (and selling) Subaru Impreza, Nissan GTRs, etc. See their Impreza Type 25 from back in the day!
https://www.evo.co.uk/carreviews/evo...d_type_25.html
However they used to have majority stake in a tuning company called "Powerstation" as well, (no idea if they are still around) who were based in Tewkesbury. A load of us used to use Powerstation for tuning our Imprezas, everything from decat and remap, suspension fitting, laser wheel alignment, brake upgrades, servicing, etc.
They always did a great job and Iain Litchfield was often around the place.
Personally and I've said this previously on here and other forums, if it were my car I'd trust having it done at Litchfield rather than anywhere else, based on my previous experience of Powerstation / Iain Litchfield.