I'm curious about range and useful load - the horrendous problem of the small airframes and turbine engines is the inability to carry enough fuel for decent range and still be able to put something in the cabin. It pretty much meant the turbine P210 and Bonanza wouldn't really work out; Piper only recently solved the issue on the Meridian by installing VGs to get the stall speed under control so now it will take three plus baggage when full of gas (1,000 nm VFR range, plus or minus 850 IFR) and no matter what, you've got to get up in the 20s to get any range and endurance.
My other area of curiousity is airframe changes needed for the speeds desired - when Riley put together a turbine version of the Cessna 421 it just didn't work; the airframe wasn't designed for that kind of horsepower - when Cessna made the 425 it only superficially resembled the 421 due to all the changes that had to be made to the airframe to handle the power upgrade. When Piper developed the Meridian from the Malibu/Mirage the only thing that wound up being common the fuselage....
I had asked about the range and the representation made wasn't bad. I'd have to go back to my notes, but the factory P-Baron carries 190 useful. Darwin was going to add fuel in the nacelle which would bring it up to 230 to 235 IIRC. You probably know more about the -21 burn rate than I do. Of course, numbers were projected on getting up to FL250 fast.
One could pull the power back to a long range fuel burn and get better range, over 1,000NM according to him.
I share your concerns about the airframe; Darwin represents it as being robust. Please understand that I've only briefly looked into this as a potential customer. Haven't been out there, haven't flown the bird and am not a test pilot. I have attached an e-mail from him and his spec sheet. I certainly welcome other folk's thoughts. Don't think I'm headed in this direction right now because my business is in the toilet, but who knows.