I hate to say it but the head is where the DS650 and EX400 have an advantage. The 5 valve head does allow for greater air speeds which make more torque but the chamber must then suffer from poor design angles. The 5 valve also makes it virtually impossible to formulate a dual cam set up.

You are right on the chamber -- it requires a substantial amount of rework as does the intake ports and pockets. It is very technical work as a slight over port will ruin the head.