“GM Turbo 400s – three-speed until 1987, four-speed after that.” Actually the Turbo 400 3 speed was used through the '92 model year and the 4 speed started in '93. My '92 was built 6/92, one of the last, it has a Turbo 400.
“Hard-to service inboard rear brakes were moved outboard in 1993.” Most '93’s had inboard brakes and were moved outboard for '94. There may have been a mid-year change in '93, but one I’m familiar with has inboard brakes and was built in November '92 as a '93 model XJS.
“A 4.0-liter six-cylinder engine arrived in 1994.” Best that I can tell, the 4.0 six cylinder started in '93 for the U.S. market in the XJS.
I’m a big fan. My '93 XJS V12 Convertible is a keeper. I’d like to have a nice coupe too.