do those figures tend to substantiate your 1.5x?
No it does not; it is only a 1.15 difference. I assume you have a recently calibrated ASI and an accurate flowmeter. This shows how hard it is to compare data, but then both our figures are for economical cruise / good engine life / LOP.
The DA42 is substantially quieter than any plane I have been in, although not by enough to be usable without good headsets.
I've previously had an unhappy experience with the UK Socata importer. Do I really want to commit myself to having to deal with a company I dislike?
Fair point, but you don't have to. TBs are currently not being made, not since about 2003, so you would be looking at a used one (and even a new one would be easy to purchase via another country, with minimal subterfuge) and there are a number being advertised which you would buy direct. The parts can be obtained via various routes but in any case if you go for something made in the last 5 years or so you won't need many; I have spent well under £1000 in 5 years on mine. Air Touring have earned a reputation for getting their currency exchange rate wrong but this isn't an issue on small parts, and anything bigger can always be sourced elsewhere if necessary, especially for an N-reg where an 8130-3 form is enough and the JAR-1 scam can be avoided so you can buy direct from the US. Nearly everything that is likely to go wrong on a fairly recent TB is made in the USA and is an off the shelf item. Only the very old planes (20+ years) need the expensive airframe parts, but airframe parts are v. expensive for all makes.
One additional factor I forgot to mention is a single door v. two doors. That makes a potentially big difference in usability. I am reasonably flexible but I know from my PA28 renting days that a lot of people have problems getting in, and the interior gets knackered as a result of people climbing all over the seats, putting their weight on the seat backs, etc. I also had a door jam on a PA28 once, and would never buy a 1-door plane after that. I've flown (in the TB20) with many people who frankly wouldn't have a slightest chance of getting out of the back of a PA28 in a hurry.
why bother saving a little more fuel so that you can go a bit slower
With LOP, you might fly several percent slower for a ~ 30% fuel saving, compared to full-rich.