Now if we could just add another LED module to the 24" fixture!
Having modified the ATI Sunpower, and running at first 48watts through it, and then 24watts of LED plus 5x24watts of T5: I think the issue is maintaining a safe temperature on such a small unit. The extra heatsink lengths on the longer unit probably does make a difference.
Think of it this way: even in your 4 tube unit, that means heat from:
1. 4x24watt T5 tubes
2. Potential 75watts of LED's.
3. T5 Ballasts
That is in the region of 180watts of T5's, LED's and ballasts all of which must be kept cool. Despite what people say, T5's do generate a lot of heat (but over a greater area, not concentrated like MH).
I've measured the PAR at the ends on my 30" long tank and even with the fixture being 24", I get VERY good PAR on the ends.
What I would have liked for the 24" models is for the LED Cluster to be SPREAD out on a longer strip. But I suppose that means more costs for ATI to make separate cluster for the 24" model.
Overall, I am very happy with my unit.
EDIT: I also think having two 75w clusters on a 24" unit would have been overkill in terms of PAR, and would push cost up too much. Also, it would change the main feature of these unit from mainly T5 with LED supplement to the opposite.