I recently used RS Tuning and it was a good experience. My car already has many of the bolt-on mods that you talk about (see my sig) and so I wasn't expecting a huge gain, and indeed I only went from 281 to 293 bhp. The biggest difference is how smooth the car is now, and the exhaust note also seems smoother, fewer transitions in tone through the rev range. Torque has been improved and nicely smoothed out and a lean running problem was solved. The car is so much nicer to drive and very punchy. I worked out that if I saved 1mpg over the next 4-6 months the tune pays for itself - result!
Anyway, RS Tuning really do know their stuff. Without boring you with the little details they capably smoothed out all of the rough edges and the car is definitely much better for it. It is a reasonably quick process ( a few hours) and you can sit and read mags and drink tea whilst they do it, or watch the process if you can bear the horror of hearing your car being revved up to 6.5-7K revs, which in a small space is very loud and worrying!
They get a double thumbs-up recommendation from me