[euromodder]

I too will vouch for the tailbag, mounted lengthwise.

It fills up the rider's wake, rather than stick out in the breeze and create an even bigger wake as saddlebags and a transverse mounted bag do.

A topcase doesn't cause as much drag as saddlebags and a transverse mounted bag.
When using a topcase, it's best to fill up the gap between rider and case with another bag.

I've never had much of an issue with the weight in the topcase being that much aft or that high up even though it was loaded well beyond the silly manufacturer's approved weight.

That included 200+ kph (indicated) on the Autobahn ;-)
And on a naked bike ...