Self-fit??? I don't know about those... AFAIK there are 3 options to
increase power:

1. A powerbox or something like it. A box with electronics fitted
between engine management and the sensors in the engine. Basically
they fool the engine management by fiddling the sensor output to get
the engine management to increase turbo pressure or amounts of fuel
injected. You may be able to fit one yourself. This is usually a bad
solution with limited results.

2. Opening the engine management box and replacing 1 or 2 chips
("chiptuning"). This can work OK but the software on the new chips
needs to be of good quality. Often retailers don't do this themselves
and just ship them to somewhere. With things like this you want to
deal with the source, so they can make adjustments to the software. No
2 engines are exactly the same and ideally the software is tuned to
your particular specimen on a dyno.

3. Re-programming the chips in the engine management box with a PC
(sometimes called "ziptuning"). Not all chips can be reprogrammed
(with Alfa the JTDs newer than 2001 can usually be reprogrammed). This
is obviously the best solution but again, you want the new software
tuned to your particular engine on a dyno.

When done professionally, options 2 and 3 can produce more power, more
torque and even possibly more fuel economy - provided you don't change
your driving habits. Good tuners also look at the other parts of the
drive train. The clutch from a Fiat Stilo 1.9 JTD cannot take the same
stress as one from a 156 1.9 JTD, although the engine is the same.

Because in that way manufacturers can produce 1 engine with software
regulated power outputs so they can charge more for the most powerful
cars yet have economy versions whilst retaining the economies of scale
by having only 1 physical engine. Everyone does it... Usually there is
some difference in the clutch, transmission or wheels fitted as
standard so the additional power can be delivered to the tyres without
problems.

Also, in fact you are (in most cases) invalidating any factory
warranty, so not all people are very anxious to do this.

Yes, had my 156 1.9 JTD "ziptuned" professionally (for 900 euros)
after 45.000km from a 115bhp rating to 152 bhp (from dyno results).
It's now at 163.500 km without any problems related to the tuning
itself. The clutch is now pretty worn though and probably needs
replacement at the 180.000 km service.

Although the engine smokes more than un"zipped" when accelerating, it
has a lot more power, reacts more aggressive to the accelerator and
sounds a bit throatier. Also, fuel economy is better now, as long as I
drive normally. Top speed has gone up to 208km/h (GPS measurement) and
accelerating from 80-120 km/h is almost 2 seconds faster in 3rd and
4th gear, 1 second in 5th. I have no regrets whatsoever.

Sorry, can't comment on that. It's a lease car and obviously I haven't
told the lease company.

HTH,

Paul
156 SP1 1.9 JTD 152 bhp