I have had several tag axel M/H's either chiped or reset by computer. The results are good and provided that you get a good chip, the difference is well worth it. For example I picked up a new Bessacarr 3 ltr Ducato last march. It has a switch on the key ring which enables it to be switched on and off. So a comparison is easy to do. For the first 150 miles I forgot to switch it on doing mainly motorway driving and returned 18.9 mpg on the onboard computer. The last 70 miles consisted off windy B roads in Wales and a long high haul up a mountain Between Llanidloes and Machynlleth. I thought that's not a good test but I returned 24.9 mpg. Having just been given an offer on an A class that I can't refuse, Iv'e easily uncliped the unit, but can't reach inside the engine compartment to attach it to the new M/H. its doing my head in as I hate waisting fuel, and the new Tag did 22.4 average on the way home. So looking forward to about 27mpg with the new lowline body. Obviously any figures depend on your own driving, but I drive within the speed limits, ( with 9 point on me licence i have too.) The name of my unit a BHP Plus Speedhawk. But I had similar results having the computer upgraded. This can however be wiped out when hooked up by your garage. so I prefer a switchable unit. Hope this is of help. Rgds' Graham.
Ps. Mine was fitted by Total Vehicle Services. Somerset 07973 116205, but they are available on the net.