I had owned previously Astra 2.0 DTI (101HP). With tuning box (manufacturer website: [url="http://www.tuningbox.com/en/home.htm"]http://www.tuningbox.com/en/home.htm[/url]) I did about 30 000 miles, then sold the car. After installation (took about 10 minutes, simple plug and play) instant increase of power and torque, dyno showed +22HP and +67Nm. It is hard to drive the same way like before, you simply enjoy the power and at that time consumption increased max 0.5l/100km (with very little smoke when I pushed to the limit). When this time was over and I started driving like before - fuel consumption was about 0,3l/100km lower than before box installation, I think the reason was that power was available at lower revs.
Of course box increases the amount of fuel, but proper box does not cheat ECU with amount of air intake. Mine couldn't, it was placed between diesel high pressure pump and ECU, there is no air intake signal in this place. But there are cheap "boxes" on ebay that cheat air intake temperature sensor etc., but they are wired differently in the car, most of the time they are universal.
The only thing I did not like was a lot of torque from low revs, this could kill the clutch, but if you are aware of that and drive the right way - all will be fine. When you do remap this will not happen.
I was lucky to get this box very cheap, in other case I would probably do the remap.
After 30 000 miles everything was OK with engine and transmission. During this time I had to replace coolant temperature sensor and clean pressure sensor, so it was nothing connected with box. I removed the box and sold it separately. It was the best modification I did with this car, and I did a lot