68V veros arent any more efficient than 1825s at a given wattage.theyre about the same. 1825 goes higher though to 325W if thats what youre into.. i think the 68V vero tops out around 240W
the ssts should be fine at 100W, you lose a little efficiency running hotter but the citis are rated to 140 Tj and you'll prob be at 85-100C tops which is what they design these chips for (run hard, high temps, enclosed luminaire).
above 100W the 2-3W it takes to actively cool a heatsink becomes attractive as you make that back in lumens when you drop the chip temp. at lower wattages its a wash on efficiency vs the energy required to run the fan. when youre running a 3618 over 300W that 3W fan is nothing, but 3W out of 75W is a lot