With the below chart, you can se the voltage drop related to current consumption. If we stay well within the 12v, we can drive the leds to their max. The max current is controlled by dimming the LED´s while monitoring current consumption, temperature and PWM.  

For a string of Photo Red´s from Cree Inc. 4 leds will produce a voltage drop of 10.4 v. (If there where 5 leds on the string, we would exceed the 12v by 1v. Using 5 Led´s on a string would max them out at 1000mA. 1amp times 2,4v is 2,4watt. Driving them to 1500mA gives (2.6v x 1.5A) 3,6watt. 3.6 x 4 LED´s =  14,4watt - 2.4 x 5 LED´s = 12watt.  With the Blues there is a even greater gab, since they can be driven to 2amp. The luminus RED´s on the other hand can be driven to 2amp and has a max power dissipation a 5W. (depending on cooling). With a forward voltage at 700mA of 2.35(datasheet) the voltage drop at 2amp is 3.35, looking a below image (source). 3.35 x 5 = 16,75, so well above 12v limit. But then again, this is full on 2amp per LED, which will generate heat. Since this concept tries to explore on how to use this excess heat, the heat is somewhat welcome, but generating that heat, we might as well make the most of it. At 1.5amp the drop is around 3v, so on a 12v string with 4 LED´s, it will not go higher. Something to take into account when choosing LED´s. 

Here is where it gets interesting. Below we have the radiant flux output of  the Cree´s. This looks nice and well distributed over the 2amp area which is max for the Blue. So at 700mA the Luminus chart is 100% and the Cree´s is 100% at 350mA. That a pain in the blibinglyblib. How should we compare. The graph looks pretty linear in both cases....... hmm. 

Cree red (XPGDPR-L1-0000-00G01) has a min 525 flux(mW) at 350mA. So at 700mA, looking at below graph, it should be 200% = 1050mW flux. 

Luminus red (SST-20-DR) has depending on which flux bin up to 990mW @ 700mA. In the low end of their bin it is at 790mW, which is strange becouse that is also seen in the mouser part. Maybe that is why the SST-20 has a lower price tag. I guess the V in the product name implies the bin (SST-20-DR-B120-V660). If we say the V type is on avage 810mW at 700mA. That is quite the difference. 1050 for the cree top line and 810 for the Luminus low end @700mA. This difference is what the price reflects.

There is the calculated output difference in "brightness". But note it is relative. 

Anyways, to sum it up. 14,4 watt @1500mA using 4 LED´s on a string. times 8 strings on each controller channel = 14,4*8 = 115.2 watt. 115.2 watt is around 10 amp @12v