down_proj is the most quant-sensitive role (38% of total error budget per EXP-0007); stacking it with k_proj protection should give strictly more recovery than k_proj alone
Adding down_proj@Q8 only saves -0.03 PPL for +0.87 bpe — diminishing returns set in fast after k_proj. The first protected role (k_proj) captures most of the headroom; adding down_proj is wasted bits at this bit budget. May be worth revisiting at 3-bit where total error is higher.