So I needed to add more LEDs. Apparently just 5x cyan LEDs cannot cover the hole left by 40W white ones, so I used all 10 cyan I had on first fixture (initially I planned to use 5 on each), and set them to maximum rated current (700mA), tuned 460nm blue die form RGB LEDs to get as flat as possible, and the best I got was this:

Much better than original white LEDs, but there is still a hole near 470nm, not as terrible as before though. It was clear this is as good as one can get with LEDs I had. If I increase current on any of them - I'll get overshoot. So I needed 470nm blue LED instead of 460nm one to fill in that hole. I ordered LEDs and started to wait...