it would have been too expensive and heavy to incorporate all the discrete requirements for Cat A systems separation,
Actually, the 427 is CAT-A certified by TCCA and EASA (VFR). It was not the CAT-A system separation that was the issue with the 427. It was the number of systems that had to be re-designed for IFR (electrical, hydraulic, etc.) and a cabin that would never make it for EMS. The 427 has its fuel tanks fore and aft of the passenger cabin. Bell presented a 427-derivative mock up at Heli-expo 2004 (called the 427s3i), and the cabin was a non-starter. There's no place to go if you want dual stretchers and enough room for an attendant to intubate a patient at the head of the stretcher. The aft fuel tank ended up being this "U" shapped thing to fit the stretcher with no access to the patients legs. With the 429, they basically started with a cabin designed for the EMS mission and built the aircraft around it. They kept the cockpit and most of the systems from the 427s3i concept.