Great questions guys... I'll do my best to answer both below...
As for how this model compares to the Rupes Bigfoot 15 and 21.
My personal preference is I like and prefer the Porter Cable "design shape" of the Duetto and the Mini. This shape of tool is shorter than the Bigfoot 15 and 21. Personal preference and all that. I'll try to take a picture of the Duetto next to the 15 and 21 today for visual context.
As for using other pads, chemicals or modifying these tools...
Think of me as a Rep for Rupes, a Rep for Mothers, a Rep for Dodo Juice, a Rep for Flex, etc. My job is to share the "brands" we carry in a professional manner and in most cases that means sticking to message, their message and that tends to work out pretty good in the long run for all kinds of reasons.
Individuals can do as they please because they are only held accountable to themselves.
I will tell you this since you ask. I spoke with Marco D'Inca and Andreas, (Rupes Engineers), Francesco, the Marketing & Communications Manager, (in control of the message), Gionata, the Business Development Director, Guido, the President of Rupes, Chip, the US Market Development Manager and Torben the Sales Manager for North Europe, a number of times in our booths plus a couple of closed meetings and not a single one of them suggested or recommended modifying their tools with a washer or in any other manner.
Thanks for asking the question though as it gives me the opportunity to share the official Rupes message.
We like to be accurate, honest as well as helpful and friendly on this forum and questions like the above are a great way to get out the official information from the manufactures of tools and products available on Autogeek.net