I do a lot of Ecodiesel and had have fixed alot of celtic tune, they claim +60hp gain but we dynoed 15 hp at most, this also reflect what was done in the tune, there is about nothing modified, no limiter have been increased, just some basic stuff increased in % across the map, etc... this is very generic.
Another thing is that for a delete, you have to leave all the unneeded sensor connected into the wiring, or hanging at the wiring Under the truck, because the tune is not a true delete, again, nothing is done in the tune to delete them, so people got check engine because of a faulty sensor that was supposed to be deleted, but they can't do nothing, so it's bull****.
That's the same with the mercedes sprinter.
Have not seen their TDI but I expect this is the same, now it's up to you to buy it or not.