Q8 is virtually lossless. The quantization is much more noticeable around Q4 and below. FP16->Q8 on consumer hardware is 2x the speed at ~99.99% the quality.
I don't have a 'source' off-hand but I recommend reading up on it if you want to learn more. A lot of models on HF show a card demonstrating the different quality trade-offs between quants.
