When I visited the Lamy shop in San Francisco, I spent several minutes patiently test driving the available pens and nibs (including all of those stupidly expensive bits I would never consider buying). I tried and I tried to FEEL IT but I could detect nothing to recommend spending huge money on Lamy's top of the line nibs.
Sales staff tried to tell me the black nibs are "smoother" but, nope. Nope. They're all the same!
I was accompanied by a non-user who quickly figured out how to use the available selection of tester fountain pens. He's an engineer, no slouch when it comes to appreciating superb instrumentation. He could discern no significant changes in the writing experience between the steel nib on the Safari, the gold nibs on the Studio and 2000, and the black/gold two-tone installed on the Imporium. He had the most fun with the 1.9mm super broad, laying down deliciously wet lines that shaded beautifully on the test paper.
I have some LX units and I like how the z52 black nibs look so I've swapped them out to some other AlStars. I also have a couple of z53 Aions with the rounded shoulders. They all write the same.
I went into the store fully prepared to spend $100 or more on a 585 two-tone gold/steel nib for my rose gold LX but the test drive quickly put that idea to rest. We agreed the reason one upgrades Lamy nibs is style, not necessarily the writing experience. The z57 makes a bold fashion statement but only to other Lamy users; no one else is going to know you spent the money.
I ride a recumbent, I play go, I use Macintosh so of course I use a fountain pen.