I found that the Miele website gives you a "Residual moisture at maximum spin speed" figure. The 1400 machines are 50%, a 1600 is 44%, and the 1800 only 43%. On that basis, I'd say the 1800 is definately not worth paying for. 1% is not a lot of water.
I think one reason why newer machine may get the clothes dryer even with slower spin speeds is that they have bigger drums. When you make the drum larger, don't you get more centrifugal force for the same spin. I remember we had a hotpoint 1200 which shook itself to bits 9 years ago. I bought a Bosch with 900rpm with balance sensing logic (which was a new thing back then) thinking it would last better if it didn't spin itself to oblivion. The Bosch although apparently far slower got the clothes just as dry and I think one reason was probably the bigger drum (as well as a longer spin). (BTW, the bosch has lasted years. It has been through 2 sets of bushes and a main drum bearing but is still going even though we've had 2 kids so it has probably done a wash every day for 9 years! We're having an extension built and the old girl has a dead spot in the motor and there is signs of slight play to the main bearing so I'm thinking of retiring her at last but she has served me well and I've found her easy to fix especially with the Bosch web site to get parts online and diagrams).