Ah.. I tried thqat aswell... but that's definetly wrong! :s
If your lowpoly is all in one SG, then your smoothing errors will be projected onto the normal, and it looks horrible... So, one should try using the smoothing groups one want for the final product aswell as when baking normals. It is true though that hard edges around a rounded corner will make the projection rays miss certain areas, but it will help you get rid of smoothing errors. So for hard edges, it is definetly better to use different SG's.
That just made me remember something I havn't said yet by the way... it might seems a bit too much having three edges at the rounded corners, but if I didn't have three edges there it'd create a nasty smoothing error.. so that's why
And about the highpoly, sure thing, just a sec! *renders and uploads images*
Cheers!