you just gotta remember that cobs are super bright relative to wattage esp when you turn them down
a 4 tube t5 can cover 8 sqft and veg up to 12"
thats just under 30W/SF at an efficiency of around 85-90 lm/w
so lets say you have the typical 4-cob 200W fixture, that you would use in an area of 2.5x2.5 to flower.
if you want to veg with that and dim that to 50% (25W per cob) youd be about 16W/SF of cob lighting but run that soft its pushing up on 170-180lm/W so thats the same amount of light as the t5 with approximately half the wattage. and youd get good coverage at 12-18", whatever height is required to cover the area uniformly.