In addition to the excellent answers that have already been given, the question indicates to me a common misunderstanding what papers actually are. They are, or at least should be, the communication of ideas, results, or some other type of knowledge that may be of interest to others. They are not badges of honour or grades or brownie points for doing X atomic units of research. In that sense, "how much work was it?", "how difficult was it?", or even "how large is the delta to existing work?" are fundamentally not the right questions to ask when evaluating whether something is publishable. If you have a stroke of genius and write down a cool idea very quickly, it's value to society is in no way diminished. Conversely, spending a lot of time on a bad idea does not make it more valuable to society.
However, as I wrote initially, it's an easy mistake to make because there is, of course, a correlation between effort/complexity and value/interestingness. Simple ideas are not often extremely valuable, because the chance that somebody else had the same idea before is high.
Apply this thinking to your question:
Are Bob's results publishable? He didn't actually do anything special, he just had more computational power. On the other hand, the results are definitely new, and interesting (for whatever reason N = 20 is the point where interesting effects are predicted).
Doing "anything special" isn't in truth what makes papers accepted. If there are interesting effects to be observed for N=20, then go for it. That said, to my ears it does sound a bit suspicious that all that was needed to resolve an open question that people actively find interesting is to throw a little more hardware at it. I would make quite sure that indeed nobody has done it and that there are indeed interesting effects to be observed (and then I would still wonder a little why nobody has done it yet, unless the paper you build upon has appeared like a week ago).
If they are publishable, what would Bob actually write in such a paper?
Summarize the original work to the extent that is needed to actually follow your paper (given the setup this may be quite a lot more detail than what you would typically dedicate to previous work - that's ok, as long as you make it clear that these are not your new results). Describe your setup and experiment. Then, if the interesting results for N=20 are what should carry this publication, describe these results and in which ways they are interesting in a lot of detail. Connect them to previous results for smaller Ns, and, if possible, go to the bottom of why the results start becoming more interesting for higher Ns.
If that does not give you enough "meat" for a normal conference or journal article, there are alternative publication venues dedicated to shorter articles, such as journal notes.