I don't know how reliable is my trial, but here it is:
This webkit side by side with Safari on an iMac G5, loading different pages, some full with heavy animated gifs, other more simple like this here MU page ;-)
I loaded each page first in Webkit as it's supposed to be faster (and to avoid the possible cache effect, which should accelerater a second loading)
Well, 'this is not a revolution' ;-) but it's generally ab it faster than Safari, though Monitor Activity shows the same footprint.
It won't turn your G5 into an Intel Mac, but it's reliable, as far as I can tell, so why not use it? :-)