"I sometimes wonder why I didn't buy a TB21 instead. It does do less MPG at low levels (lower engine compression ratio), has about 2x the engine fund (bigger overhaul cost, due to a very rare IO540 variant), but with full TKS it has much more mission capability."
DA42 has TKS as an option. It works very well indeed, better than other TKS systems I have tried.
Others have misquoted perfo of DA42 - in fact you will see TAS155kts at F100 at 70%, burning 4.9USG per side with the latest 2.0 engines.
Finally the practical platform of the DA42 is F180, but yes, they have tested one to F250 (or thereabouts) at which point one of the engines stopped, presumably because the ECU could no longer work out what was going on. It restarted quite happily at lower altitude and there was no permanent damage.
To me the biggest advantage of Diamonds is burning JetA1 - not just because it is cheaper but more importantly because of its very high flashpoint. No Diamond has ever caught fire on crashing (and few have crashed and then due to pilot error). I once had a major fuel leak on a DA40 - on landing at the diversion I took the cowling off and started up and it was spraying very large amounts of JetA1 directly from the common-rail high pressure pump onto the turbocharger. I had flown for an hour or so (across water) to the diversion in that condition.
With most Avgas planes, if you crash or have a fuel leak of that magnitude you are lucky if you don't have a fire.