More is usually better when you accelerate in a straight line. It will give you equal power on both wheels. But in corners it can make the car very 'snappy', so you suddenly get oversteer when one or both wheels start spinning. Lowering the diff makes it more predictable and usually easier to handle. When you're not spinning the wheels, lower diff gives you a bit more oversteer on exits, which can be useful, especially on a lot of exits at Spa.
Wish you'd post your setup Or even onboard vid. Your times in the Abarth are insane...