As Mark has said, Unichip, while not being the best piggyback out there, will be fine for NA modding. As long as the mapper is good, that is what is key. £400 seems a good price for the chip and mapping, RSTuning are offering reflashing and remapping of the original ECU for about £500 +VAT so its a good deal you've been made. Just dont expect too much out of it, its not an FI car!
As I said before the key is a good mapper, they are the ones in control of the results. Its a good price and if it gets around the speed limiter too I dont see why not. Just dont expect to be able to reuse it if you want to go FI with decent results.