Do figures exist regarding the speed difference compared to MiamiDX?
There have been several threads on EAB and here with transfer speed results. Miami and MiamiDX have consistently done poorly. Genesis/EasyNet/AmiTCP 4 has generally been the fastest. The Roadshow was a little behind Genesis but may use less processing power and may have been improved.
What do you mean with "semi-modern TCP/IP " (with view on MiamiDX)?
What do the others offer that MiamiDX lacks?
MiamiDX is now one of the oldest TCP/IP programs as far as updates. Dial up was still popular at the time of the last update. It was fairly advanced for its time but it was never finished, some functionality is buggy and it is slow. The GUI was the best at the time but now Genesis and EasyNet also have a nice GUI. If MiamiDX is working fine with your current setup then there is not a big incentive to upgrade but I wouldn't recommend it for people needing TCP/IP software anymore.