I haven't tested black flat yet, but conclusion is that LEDs with very small die and small heat pad area really need copper DTP. Efficiency of LEDs drops at higher current because of two things: temperature of die, and current itself(why, still not fully explained by physicists) . For most big LEDs second cause is dominant at high currents because lumen stops increasing even if die temperature isn't even close to 150C, which is maximum allowed by most manufacturers. Maybe best example is Luxeon V with industry lowest thermal resistance per mm2 of die (0.8C/W,4mm2), it tops at around 9A, but if you calculate die temperature at 9A (LED on CuDTP, room temp. 20C), it is only 20C+ 9A*3.55V*0.8C/W=45.5C , not even warm in LED world. So second cause is dominant. This means even if you put such LED on non-dtp board which would cause for ex. 10C higher die temperature, that's still only 55C, difference in light output would be hard to detect.
On the other side it seems temperature effect on black flat is considerable, simply because power density for tiny 1mm2 die is very high. Temperature of die with copper DTP at room temperature,5A is 20C+5A*3.9V*4.3C/W=104C , this is quite high, if you take fact that typ. flashlight would have 40-50C temp., then die temp of oslon is very close to max. 150C. That's why every additional deg C caused by non-dtp board would have visible impact. Problem with black flat is small heat pad (3.5mm2, but it's questionable how good is heat distributed over that 3.5mm2 inside package), if black flat is in XM-L sized package, difference between DTP vs mosX would be much smaller.
As I said earlier, I designed mosX Osram/luxeonIR boards for infrared LEDs, where power density (~ max 2A) is much lower than with black flat, I expect no visible difference DTP vs. mosX in that case.
EDIT: When soldering wires to mosX boards, best method is to put some solder on iron tip and try to put some solder on one of the big pads. At first nothing will happen, but as board heats up, solder will melt over pad when board reaches high temperature. So it's important to hold tip on pad steady for some time. After board is hot, sodlering wires on all pads is easier. Board must be on piece of thick paper or some other thermal insulator.