If you mean the difference between the Evo Abarth and other Evos, then it is the turbos that have a different turbine (internal) design. This gives the Abarth a significant increase in boost at a higher RPM. This makes it less suitable for pottering about, but deliver more power at sustained full throttle. The programming of the valve timings will be significantly different with a bias to power also. I don't have any Evo engines, so I haven't researched the fine detail of the parts, but a quick look suggests that apart from turbos, there are few or no significant differences between Evo FIRE turbo engine internals apart from turbos as valve timing can simply be adjusted in the ECU for different specs.
Without the benefit of MultiAir, previous Abarth T-Jets, Bravo 150 and high spec Alfas had higher lift (I call them Abarth) cams as well as the larger turbos to give them an extra edge. Again, this had the effect of raising the power band up the rev range. Good power can be had from 1500RPM in a 120 T-Jet, but subjectively, I would say it is 3000RPM in a 150 or Abarth T-Jet. It's something to think about if you only very rarely want to cane it.