David & Anne, I fitted one of the add on type boxes, I was rather sceptical of the claims as I am a fully qualified motor engineer, however I know from my time spend running the exhaust emmissions lab for ERA and later Aston Martin that once you've tuned for power you then back off slightly for economy and then you have the task of trying to meet the current exhaust emmision requirements.
Anyway in answer to your question, I've never tried the motorhome flat out so can't comment on top speed, however there's a hill near me on the way to work that if I hit it at 55mph I'm down to 45mph at the top (should really change down!) with the chip box which only takes 2mins to fit to the fuel rail I am now doing 48/49mph at the top! So there's a REAL improvement to torque.
The vehicle now runs a LOT smoother at the lower revs, at the time when you would change down as it starts to labour; you can now run even slower making it a lot more relaxed in town and can hold a higher gear for longer.
Fuel comsumption is unchanged, I did think if you didn't use the extra power then there could be a gain in F/C but I've not seen it. (I get 24mpg using it on short trips to work and back, and 27-29mpg on normal runs if that helps).
Like others have said, I was surprised at the lack of power from a 2.8jdi but the dealer said mine was the largest motorhome made before they then fitted the lower 5th gear due to complaints about maintaining motorway speeds when laden, also THEY recommended re-chipping as they now make a 2.8 hi-power which is 160hp - 127hp std. My chip box maker states 152hp which seems a fair claim.
It was an end of line ebay bargain... £37!!!!!!!!! So if you can get a box for a reasonable price then I would FULLY recommended it, but make sure it's an adjustable one! (Chap at work bought a non-adjustable one off ebay for a 1.9tdi Passat and although it went like poo off a shovel it mis-fired at low revs!). Come back to me if you need further help/advice - Good luck!
