I've see the Roundabout advertised, but never really wanted to do 25 miles over those hills! It was 17 miles on them that buggered my knees back last August! I think it's evil making you go right back down to the valley floor between Waun Fach and Pen allt Mawr!!
You're right that the ridges are boggy, esp. that between Crug Mawr and Waun Fach, and for that reason I'd personally go for the Keen Targhee Mids or another light Mid boot, largely because I also wear custom orthotics for the same reason you do and - given that mine at least were horribly expensive - I don't want to get them full of the boggy goo you find up on the Black Mountain so often.
I have Keen Arroyo sandals which take the orthotics brilliantly, and the Targhee Mids are on my list of mid boots to try out once I get my knees sorted.
Oh - and good luck with the roundabout!