Second what PeteM wrote.
I have M20 and M15 (top of line version) and have used them for years. I can likely answer any of your questions. I also have and use extensively some Husqvarna top lever hammer doubles, so, can make direct comparisons. FYI, I much prefer M48, 51, 52, etc. hammer doubles over underlever ones for serious wing shooting, mostly because stock dimensions. However, I have done quite well with the underlever ones too. The top lever guns seem to have been heavily influenced by designs produced by Sauer & Sohns, etc.
Niklas
