The final outcome of my experiments for an outdoor text display work fine, I am using SAA1064 chips multiplexing at 50%,
I left the board facing the sun for 3 hours, and the front was too hot to touch ( the customer wanted black ! ) but I fitted a fan inside and the air coming out the ventilation vents was less than 40C.
My LED supplier can now sell me LEDs 5 times brighter than these, so I will go normal for multiplexing for the subsequent boards !