BTO might make Sense I suppose. For people that don't use cad and so forth the consumer models are a better choice however. I for one prefer the non-pro because of better value. And for my use probably better performance as well.The top half of the 27" iMac line is very much a work a day pro machine these days, well some time really. It really could use Pro level graphics as more and more big interface software can really use GPU processing days.
I know a few offices buying iMac 27 was CAD/BIM and it's Always a BTO with two main changes up the GPU and switch to extended keyboard. Well 3 more RAM (but that might be done after market).
AMD or Radeon either way pro level would be good.