Having personally wheeled both setups, I can tell you even with a diesel, 5.38's provide just enough power. As far as performance goes, I saw no significant difference between 40 and 42. If anything, with 42s, you need to go higher and you'll still rub big time unless you extend your bump stops. Of course, on a JT, trying to keep your rear pinion from binding at a full droop is more of a challenge as well especially if you're running coilovers.
That said, I am still running 42's with my 3.6L and they look great but I'll most likely drop back down to 40's when they wear out.