I expect your manifolds to flow better than Mike's. Mike is fully aware that performance sacrifices were made in his design to allow a simple bolt up installation. Yours will likely flow better, but to some that is irrelevant considering your requirement of non-standard parts.
Greg's soon to be released downpipes will bolt right up, and to some that is more important. Not everyone has the time, money or desire to fab up custom parts, or be locked in to a single source supplier.