I've been running the stock front O2 sensor in my downpipe for over 2 years. It actually reads pretty close to my LC-1 down to around 11.14. No ill effects, stumbles, or adverse behavior. It reacts just as quickly as it did in the manifold.
FYI, the JDM twin-scroll turbo setups run the front O2 sensor in the downpipe as the OEM location.