I've got a pair of 23w CFLs on my 40BR, and If I forget to swap out the DIY CO2, its really easy to tell.
I don't think that with 2 39w T5HOs, no CO2 is even going to be an option.
That being said, the 2x39w T5HO produces about 12 times (7000 lumens) the light that the LED fixture does (600 lumens). T5 will vary with reflectors, etc, but that fixture is going to be drastically more light than the LED setup, whether or not it has good reflectors.
Not all LEDs are the same.