I haven't done a thorough or systematic comparison, but I'd expect slightly slimmer but similar results with Sinatra apps given the 80MB figures you quote above, the rails processes were around 120MB on average, Go 6-10MB and scales without extra processes. Obviously the smaller the framework you load for each process, the less memory you're going to use, but there are also architectural differences in the languages which seem to add up to significant memory usage in the case of Ruby.
Because to scale ruby you'll typically add more processes, and to scale go you'll just let it start another thread (the http server does this for you), that difference adds up to a significant difference when you have a lot of traffic.
That's not to say that Ruby is impossible to scale and I love Ruby the language, but it is a bit slower and a bit more memory intensive than Go.
Also, you can move to JRuby if you want a one-process/threaded model, which will get memory-conservative very quick. A JRuby thread will quickly get you into the ranges that you quote for go + the base cost of the JVM.
