Sorry, but I could no help this one. Reading of STI 5600 / R700 manual (disclaimer notes):
https://si.shimano.com/pdfs/si/SI-6KD0C-002-ENG.pdf

"Use a [sic] frame with internal cable routing is strongly discouraged as it has tendencies to impair the SIS shifting function due to its high cable resistance."

Since, Shimano moved on to the concealed cable housing for not only brakes but also shifting and introduced the right angle kink to the shifting cable right in the lever, also there's no mention on the internal cable routing at all (4700-9000):
https://si.shimano.com/pdfs/dm/DM-ST0002-05-ENG.pdf

So, what has changed since? Are the cable housings space tech now?