TOSNA does add less yeast assimilabe nitrogen (YAN) than the nitrogen requirements for most yeast, and yet it seems to work as it has been used countless times with success. It may be that organic nitrogen is truly more efficiently used than nitrogen that comes from ammonium ions (in DAP), or perhaps there are other yeast essential nutrients (rather than nitrogen) that support lower nitrogen requirements. I still haven't seen enough data to feel that I understand what is happening, but I won't argue against using TOSNA as the empiric evidence that it works is substantial. With that said, I usually use more that the TOSNA amounts in my batches, and generally find it works well.
Trying to use substantially less than the TOSNA amounts seems like a neat experiment - sort of a how low can you go and still get a good result test? At some level, the lack of nitrogen, lack of other yeast support factors, and lack of buffering are going to leave you with a batch that is being fermented like a show mead with no nutrients - like in the bad old days - with a long, and painful fermentation that is high-risk for getting stuck.