I would say that 23rd ggp is too far back, even with pedigree collapse and recombination factored in... The lower the probability of the match, the smaller the segment is of shared dna. A low probability 5-8 cousin match could easily be a 11-12 cousin - especially with pedigree collapse. Personally, I have a good number of low/very low probability 5-8c matches that are really 10-12 cousins. Since the tree comparison tool only looks at the first 10 gens, I always look for something outside the 10 gen lists when looking at low/very low 5-8c matches.