> why, by default, hardware checksum and TSO are not enabled?

Good question ... from my viewpoint - as a guy who gets
yelled at when drivers don't work - because it makes the 
driver work out of the box, even if we incorrectly did 
the auto-detection of whether or not this particular nic 
variant REALLY supports these features or not (and if so, 
if they really work according to the data sheet).

I know, this somewhat conservative approach does not
yield the best benchmarking numbers out of the box,
and requires some command line options for best 
performance for any particular configuration.

It's a bit like having a Honda civic instead of an Italian
sports car - it's not as fast, but it starts every time :)

--
aboyd