Top tube downslope angle

are you a pie-eater? maybe that's why the builder wants to go compact? if you are a lightweight then maybe you could discuss with him about backing off the angle a bit?

Not particularly - but it's a large frame.

my cynical nature suggests the builder doesn't want to source a longer tube than the ones he has in stock :-)

We've fiddled about with the head tube length quite a bit, as I've ummed and ahhed about the length of forks I want.