I've heard the theory that stem length affects whether the bike feels stable or twitchy. But on my own bikes it depends on frame size and overall fit.
I think my Ironman is 57cm or 58. The original stem was spec'd at 125mm, although I don't know whether that was c-t-c, end to end or something else. When I measure stems they never match the specs, nor does any combination of measuring methods. Anyway, it felt stable, especially on fast curves, but I felt too stretched out. A 90mm stem feels much better, and took only a little practice to feel natural.
My 56cm '93 Trek 5900 came with the original 140mm Ibis titanium stem. Elegant little thing, but I was way too stretched out and my neck ached after every ride. I just swapped to a 90mm last week and it's much better. But the shorter stem also feels more stable. With the longer stem I really had to pay attention when checking over my shoulder to be sure I didn't drift off line. With the shorter stem, no problems. And it feels more stable on fast curves. I also switched to compact drops with shorter reach to the hoods and drops, both much better than the original.
I suspect the longer stems were spec'd for younger, fitter riders who could use a more aggressive position. For me it was just torture. I'll never be fast enough for a slight aero advantage to translate to faster speed in the real world.
I doubt switching to a smaller frame would work for me. I'm 5'11" with somewhat longer than usual thighs (shorts that fit knee length on other folks ride up mid-thigh on me, like 1960s short-shorts). A 56-58cm frame feels about right. I have one bike that's marginally too small, a Globe comfort hybrid I use as an errand bike. It's spec'd for riders up to 5'10". It always feels cramped no matter how I adjust the handlebar, stem, saddle, etc.
I've ridden only one newer bike with the currently vogue shortish top tube and deeper drop from the saddle. It felt odd at first but okay after a few minutes. I didn't ride long enough to know whether it would be comfortable on longer rides. It seems to substitute certain advantages and disadvantages without actually fixing every bike fit problem. The arms aren't as stretched out, which may be more comfortable on longer rides. But it doesn't necessarily look any more aero in terms of rider position, which may be why some pros take risky and aggressive positions -- sitting on the top tube -- on fast descents.
Dunno. If I'm miserable after 50 miles, it doesn't matter how the bike looks or rides or theories about bike fit.