You can go old-school and generate growth curves.

It is pretty straight forward. You setup a series of plates, then trypsinize and count cells at various time points. The resulting curves provides an easily comprehensible graphic of differential growth between various cell lines.

Oh, I forgot to mention we work on living drosophila, it seems a bit more complicated this way. We want to compare the cell growth capacity but beyond this question are other questions : Are cells growing faster or are they growing bigger ? The amont of ribosomes seemed to be a good measure but we don't know how to do this. For cell culture, of course it's a good idea, thanks

I'm not sure if the number of ribosomes will be an accurate reflection of cell growth rate or not, it might also represent protein production levels and not growth, depending upon the cell type.

Thanks for your answer Michael, that's the point. We found an overexpression of ribosomal proteins when overexpressing our favorite protein in drosophila wing discs and we would like to quantify if cells can grow faster or bigger or even produce more proteins, maybe we would need several techniques to answer these questions. I thought about Flow cytometry for size but for the cell cycling I don't know and neither for the protein production, that's why I first thought polysomal fractions were a good idea.

The only other thing that comes to mind is directly measure a population of cells from photomicrographs. You can then compare the averages from control flies and your overexpression line. Good luck, it sounds interesting.