When Sierra first started the FJ44 conversions in the early 2000's, it made more sense dollars wise. There wasn't much competition for the money, as far as speed and fuel flow went. The only thing close were the early model CJ's, but they were 20kts slower, and had less range. The CJ2 was new at the time, and had comparable numbers to the Sierra FJ44 500/501's. But the CJ2's purchase price was about 3 times more than what you could buy a converted 500 series for. Hence the Eagle II/Stallion was a viable alternative. Now, with the oldest CJ2's being around 10 years old, the price spread between them and a converted 500 series isn't that much. I suppose if someone has an old Citation where the Pratts have reached TBO, and they want to keep the airplane, then maybe the conversion is worth it. But buying an airframe that old, and then putting that much money into new engines doesn't seem worth it.
The Bravos, Encores, etc. are in a different category operating cost wise, than Williams powered Citation products. Check out the burns of a CJ2 vs a Bravo at comparable cruise speeds and altitudes. Not to mention overhaul costs. That's one reason why Cessna stopped production of the Bravo (and soon, if not already, the Encore) in favor of the CJ series.
The Hawkers are a huge move up for someone considering 500/525 series Citations.