A so called boutique custom will never have the performance of a top Al bike of any name brand manufacturer.
Reason is simply, one off's from a small mfr comprised of more uniform section tubes aren't optimized for vertical compliance and lateral stiffness like a top non custom production bike is. Boutique companies can't afford the R&D for the computer modeling and copious testing of frame section combinations that work after many generations of refinement based upon given frame section combinations developed for each frame size. An example is a company like Seven who makes a custom bike with geometry identical to a Roubaix based upon customer request. They are out there btw. It won't have the refinement of a Roubaix with countless hours of development and now 5 generations of design.

As to geometry, unless, an NBA basketball player or a little person...virtually anybody else can find an excellent fit off the rack unless exceptionally freakish...because of all the geometries being available in Al....from CAAD12 to new Allez now available in endurance geometry and race fit...including the H2 Al Emonda which is in between.