Where do comercial aircraft travel in relation to the Jetstream? Above if they can or well below?
If it is a Tailwind - Right in the middle, as close as possible to the core to obtain the maximum Tailwind benefit.
If it is a Headwind - Normally below is the far better option, given that above is usually well above Optimum Altitude.
The Japanese Jetstream? - I've no idea of the mechanics of it, but when it's rough, it's very very rough, with very strong wind shear. In my experience, the rest of the world's jetstreams give little, light, or moderate turbulence. The Japanese version can be from severe to extreme. My worst encounter with the Japanese jetstream was on a par with a Cb entry.
Regards,
Old Smokey