Thanks for the replies. I want to shorten these extensions because the headtube is already very long by the time I apply the other geometry I want. The uncut steerer on the fork I want to use is only 300mm long. Once you account for headset stack and stem clamp space, this does not leave a lot of extra length to work with before I run out of steerer length. I could fix this by buying a fork with a longer steerer, but I don't like the available options. (This is a road disc bike.)
If any of you believe I'm thinking about this the wrong way, please speak up. I'm thinking I will lower the top-tube, seat tube junction by a few mm while maintaining the top tube angle to give myself a bit of extra space to work with on the headtube extensions.
By the way, the reason for the long head tube is that I'm very tall--6'8". My fitter recommended a 234mm headtube.