I used Red Dot. Yes it's worth it. If you have a full exhaust system with Decat you should gain around 20 BHP.
You'll shave at least a second off 0-60. I recommend chatting with them first over the phone. They are the cheapest, and you get a further discount with being a member of the club here. Extra 10%. If you need, they can also do brakes (at 40% off) But you have to travel to Watford.
All mapping is done on rolling road. You get before and after graphs etc.. Plus the guys give the car a good diagnostic first. You can that way get a custom map to account for the exhaust.
It was worth it on my twin spark anyway.
OA
Angel and Celtic do a mobile service.