Interesting isn't it - I'm learning that any form of Boating is not an exact science
According to Crewsaver the issue with drysuits and boots is that the air in the boots raises the height of your feet pushing your head further back towards water level.
The new 190n design is really neat - its designed to turn you over if on your front due to the inflation bags over lapping - it also sits much higher up your body keeping your head further above water level and therefore better compensates for heavy clothing/drysuits and air pockets.
The implication from Crewsaver was that the 275n in its current form doesn't really improve your chances of survival - regardless of clothing and in many cases can impeed survival - but quote "its all a matter of personal choice" which when you are new and learning is very hard to make. Once assumes more is better but in this case it would seem not to be so clear.
Therefore the 190n to me seems the most perfect solution and the inference was that those with RIBs would see the 190n become a common choice - who knows - I get the feeling its a discussion point that will never be clear or won