...But, on the other hand, in this case the realization seems to be
wrong: probably still all locally created packets will be 
treated the same - or I miss something?

Jarek P.

The TCP layer will generate TSO packets based on the kernel socket
features associated with the flow.  So if you have two devices, one
supporting TSO, the other not, then the flows associated with the
non-TSO device will not have their packets built for TSO.  This has no
bearing on the device supporting TSO, which its feature flags will
propogate into the kernel socket for that flow, and cause any TCP flows
to that device to be TSO packets.  So in a nutshell, disabling TSO is on
a per-device level, not a global switch.

-PJ Waskiewicz