There may not be any more detailed data available.
The scale of the standard topo for the US is 1:24,000. USGS first produced 1 arc-sec gridded elevation data from these and more recently using more refined algorithms produced 1/3 arc-sec NED. Since Mexico's (and Canada's) standard topo scale is 1:50,000, 1 arc-sec is a sensible limit on what could be produced.
Check with the Mexican government to see if anything more detailed is available.