Thanks to those who took the time to respond with valuable perspectives. I wanted to revisit this and offer up the experience I’ve had since I posted the original question in hopes it helps others who are looking for more info on this topic.

I’ve since talked with a number of consultants and posed this same question to them. They all told me wearing a nice suit was fine as long as I wasn’t flashy with jewelry, accessories, etc. I decided to wear a nice designer suit to my interview and received an offer. I’m sure my suit had little to no direct impact on having an offer extended, but I felt that it presented me in an appropriate and professional manner befitting the occasion, and I was therefore more comfortable and relaxed than I might have otherwise been. Of course, if I walked in dressed this way and acted like a tool or presented myself as that kind of individual, the outcome would likely have been very different. However, wearing a nice suit, independent of distasteful or obnoxious behavior, is apparently not a disqualifier.

 

That's because you don't know what a nice designer suit is. The difference is subtle but noticeable. Materials are better and and fit is customized to you. Someone not looking wouldn't notice but someone in the know may appreciate the attention to detail.

 

I would agree that there's no upper limit on quality and fit for an interview suite. Just be careful straying too far from the baseline conservative design. For reasons I can't recall, I wore a green tie that I liked to regional interviews (back when those were not a slam dunk). By the time the majors called, and I had invested years of sweat, blood, and tears I didn't take any chances and did the red tie/white shirt thing.

 

I know a lot about suits.
We need to first define what a "designer" suit is.
People who wear nice off-the-rack suits don't call them "designer" suits. I'm talking Brioni, Kiton, Isaia and so on. If this is what you mean then ok. The "designer" suits are mostly slim cut short tail crap that just looks stupid, even though people wearing them may think they look trendy and that they are "customized" for you. I'm talking crap like Hugo Boss, Armani, Zegna, Ralph Lauren etc. There are a few exceptions from these brands (like RLPL). Most of these are made from average fabrics (Reda, V. Barberis Canonico and such. Ok'ish, but far from the best), and their construction is cheap (fused or half-canvassed v. full horsehair canvas construction, machine sown seams, sleeves not applied by hand, things like that).
If you are going to spend a decent amount of money on a nice suit, avoid all the "designer" stuff and just go bespoke (bespoke, not the made-in-asia MTM stuff. MTM is a good option if you know what you want and how to make it happen). You'll get something that fits you well, will last a lifetime, and looks good, always.