Is it possible to increase CRI of LED lights/strips by using different ones together?
OSRAM published an exhaustive document explaining the limitations of CRI and the case with deep red. It also describes the mixing of white LEDs with amber LEDs to obtain a high-CRI (including R9) light with high emission efficiency.

http://ledlight.osram-os.com/wp-content/uploads/2012/02/OSRAM-OS_WEBINAR_HighCRI_06-26-12.pdf