That was My first thought, not sure if those TF's come with compatible seats, and the expense to switch if not isn't worth it with other options there for this app. I agree 100%, lighter is going to improve output, the benefits are greatest with aggressive lobes & high ratio rockers or race rollers, just saying "flat tappet" isn't enough of a description. Specs please.....
As far as the factory hollow-stem issues, these aren't old mass production units, & I wouldn't do the exhaust valves on this anyways. The heavies are the intakes, it'd take Ti to get the intakes down to the weight of the SS exhausts as is. The exhausts are the likely-to-fail/sieze during an accidental lean event, leave 'em alone.
