Part of the reason we didn't see much in the way of YoY improvements for Bulldozer, is that AMD almost immediately abandoned it and threw resources at Zen after it launched.
Steamroller did see 30% IPC improvements over Bulldozer (all the design work would have been done before they switched to Zen), but AMD canceled the full FX version, and only ever shipped the APU version of Steamroller (with only 2 modules, aka 4 threads).
If they had shipped a Steamroller FX cpu, the generational improvements would have looked similar to many of the generational improvements that Zen received... but didn't really matter as Bulldozer started so far behind.
Steamroller did see 30% IPC improvements over Bulldozer (all the design work would have been done before they switched to Zen), but AMD canceled the full FX version, and only ever shipped the APU version of Steamroller (with only 2 modules, aka 4 threads).
If they had shipped a Steamroller FX cpu, the generational improvements would have looked similar to many of the generational improvements that Zen received... but didn't really matter as Bulldozer started so far behind.