length can also be a factor, as my 48" llight with 8 bulbs puts out twice as much par as 8x24"bulbs in my cube

You are correct that the 48" fixture will have a higher PAR output than the 24" fixture with the same amount of bulbs. Length however is not the determining factor as T5's will produce identical PAR ratings along the length of the bulb no matter where you take the measurement. The reason why the 48" fixture will produce more PAR is due to the bulbs being 54 watts as opposed to 24 watts. The same increase in PAR would also be seen if you were to overdrive the 48" T5 bulbs on an icecap ballast.

Have you done PAR testing on your two different fixtures? I'm always curious to see results from different T5 fixtures.