Realistic improvements

I know that the aim of zwift academy is to get faster times on the 3 segments but what kind of performance increase should I be expecting/aiming for?

Really hard to answer without knowing what your base fitness is and training history prior to the academy. First timers that have never done structured training might see big improvements, others might see hardly any at all. It’s comes down to “marginal gains” for a lot of cyclists. ZA isn’t the best training plan anyway, too spread out over 8 weeks.