Now, I bet that with 6GB of RAM now coming to all iPad Pro models, instead of just the sub-1TB ones, this means that iPhone will be getting 6GB of RAM this year. And the 5G iPad Pro(2021?) will get 8GB of RAM.
We won't know exactly what changes Apple has made to the A12X to get the A12Z until later when someone does a benchmark and/or a teardown. Personally, I bet Apple has increased the max clock rate(A12X was 2.49GHz on the big cores. A12Z could be 2.7 or 2.8GHz), likely also added more cache. It's also possible that Apple is using a tri-cluster, so they'd have 4 big cores, 2 medium cores, then 2 little cores. The A12Z medium cores would have performance somewhere between the A12X big cores and small cores.

Apple iPad Pro 2020 AnTuTu Test Suggests A12Z SoC isn't Much Different from the A12X SoC - MySmartPrice
Apple unveiled the new generation iPad Pro (2020) yesterday. This new iPad Pro comes with an all-new chipset, the A12Z Bionic, which, according to Apple, provides the iPad Pro with more power and faster performance than most Windows PC laptops. The brand, however, hasn't provided any performance...