15-20% on economy without any mechanical changes?!

The chip box will advance the timing slightly which might improve the consumption slightly (<5%). That's it.

Damage? Depends how much extra fuel the chip demands.The crafter is quite a big vehicle and VW chose to rate the engine at a very low spefic output . Since most manufacturers do exactly the same, there's a pretty good reason for it. Copmmercial vehicles are not passenger cars.

If it looks too good to be true...

The link doesn't work.

You just don't see big gains in efficiency from plug in boxes, I'm afraid. Getting more power is easy. Getting the engine to live with more power under sustained use is touch and go.

It's a bog standard fuel dump. It modifies the signal from the ECU to the injectors. So if the ECU requests, say, 50mg/stroke of diesel, the tuning box will increase it to 60mg.

You'll get an apparent gain in fuel economy on the trip computer because the engine is injecting more fuel than it thinks it is. But a brim to brim measurement should reveal no change. Increasing fuelling without modifying the turbo map will increase EGTs and might cause smoking. The increase in power and torque is genuine-but it isn't free!

Overdrive Technology (TM!) is a nonsense. Overdrive refers to gearing, nothing to do with the engine!

It's your call though.