I mapped the KLD of KV cache quantization for Qwen3.6-35B-A3B and Gemma4-E2B QAT

TL;DR version

• q8/q8 is nearly free on both models
• q4/q4 is useable on Qwen and catastrophic on Gemma
• turbo4 is sometimes slightly better, sometimes slightly worse, than q4_0
• turbo3 and turbo2 allow compressing the cache to unprecedented levels - but you'll pay dearly for it
• K is sometimes more sensitive than V, sometimes less, sometimes they're symmetrical

Full analysis

Nuance, caveats, zoomable plots, and the software to replicate these plots with any model:

https://github.com/crusaderky/pixi-llm-recipes/tree/main/perplexity#readme