In answer to the question, the consensus appears to be that the ASPH has a vastly improved wide aperture performance compared to the pre-asph. The pre-asph was regarded by many as being in need of an update, not because it was badm but because of the performance of the latest ASPH lenses from Leica. I have not seen a review of teh 50 ASPH lux that has not felt that it is as good as it gets.
If I was shooting wide open (1.4-2.8) and could afford it, It might make sense. If I was shooting more stopped down I would save money and buy something else, slower and cheaper. The 50 1.5 Nokton has been seen by many reviewers as the equal or just superior of the older pre-asph 50 Lux and therefore cracking value for money and a great lens irrespective of price.
From what the MTFs say and what reviewers say, I dont think you would need to be a genius in the darkroom to see the improvement with the ASPH wide open, but stopped down I suspect most differences would be in 'subjective rendering'. I for one can see a visible difference in my 50 ZM planar from f2-2.8-4. As the APSH is supposed to have superior performance on centre at f1.4 than the pre asph a couple of stop down, I think you probably would see it. I am sure there are users here who have owned both and will be able to tell us. As for whether any of this matters to you, only you can say.