A little update, the 4-2-1 punto manifold was not a straight swap. I had to make a different final bend before the central pipe and create a custom bracket near the gearbox. My panda also had bigger studs than the punto, so i had to enlarge the manifold holes. I've put a central 200cpsi cat and silencer, and a fiat 500 abarth original dual exhaust. The car is 10x better now, revs up nicely and consumption has improved a lot. (I've never seen 18km / l at 3000rpm). The noise that the exhaust makes is beautiful, at idle it makes the typical sound of abarths but it is not loud (thanks to the additional silencer). From 3500 rpm the noise becomes much louder. No lambda errors so far.
The weak points are: the abarth exhaust tips are too protruding outwards and too big for the lines of the car. I'm not sure if the catalyst is working because it smells awful from the exhaust, and I don't know if it's his fault or the scraps due to the cuts and 4 welds made on the exhaust line.
Now I do a few kms and decide whether to continue or to go back before I spend more money.
If I want to continue, I'll have to replace the abarth exhaust tips, change the central silencer and cat for better ones.