I've done a fair bit in this field (kind of a hobby) and worked with a number of tuners here in the EU and North America. A GOOD and i stress the word GOOD remap will be alot more than £170. You would normally be paying circa £300-£500 dependant on supplier for this application.
There are plenty of knock off maps circulating, a quick google search will show this can be done for less than £100 with a cheap remap tool, laptop and a flash file. The only thing I would say is you pay for what you get, as with most things. The large companies will provide the below in a diesel remap;
1. Cold start protection.
2. Overheat protection
3. Boost limiter
4. Fuel pressure monitoring @ the fuel rail
5. EGR burn modifications (dependant on the application)
6. Increase in MPG
7. Modified throttle response
30% is a big claim although also achieveable. The Vauxhall 1.9 CDTI's comfortably go from 150bhp upto circa 190bhp with no problems. This is on standard hardware of course assuming the clutch is also in good health. The modern BMW diesels react really well and you would see circa 25% increase on them also with no problems, sometimes even more.
If you research thoroughly and talk to the company doing the work, and are happy with both of these then by all means good luck and enjoy the extra ooomph
