New owner, age old question :-)

[blue2000s]

I don't have any concrete evidence, but I'd guess that they've improved breathing over the years with port, head, valve and valve timing changes. Continuous variable valve lift and timing was introduced in 2003 which certainly aided in power output.

Notice that power went up by 12% but torque only went up by 4%, meaning the torque curve either moved higher up in the RPM range or it widened, meaning better breathing either higher up or across a wider RPM range. This, to me, points to the improvements in valve timing.