Apruneuk,
We operate CJ's, to all intense and purpose similar to the mustang operationally. We have no great problem with slots and operate mainly from the South East. We operate normally at about 38,000ft, no resistance from ATC (think that is an urban myth), however I have my own views on non-TCAS aircraft in RVSM airspace. We have unpressurised cabin space, and fly 800+ hours a year, so I assume that it is not deemed an important factor for the paying clients.
We had a choice, Mustangs (unproven, range limited, but NEW) or CJ's (tried and tested, better range / payload and a toilet with a curtain, but Older).
By the time you factor in finance, depreciation etc both aircraft cost the same, I think the Mustang will make a good living going back and forth to LBG, DUB, IOM etc, and the CJ's make a good living going back and forth to NCE, GVA, CEQ etc.
Phil