If you are perfect binding then, yes, you will lose about 1/4" in the spine, and maybe more, visually, because of how the pages will "curve" into the spine, so I think if you want the "appearance" of a gutter equal to the outer margins, you should be fine.
For our clients (I used to work for a commercial printer). we would bind up a dummy in the spec'd stock so they would get a sense of how it felt for weight, thickness, and how "flat" the pages will lay when open to a spread. You don't want the perfect bind to "crack" when laid flat, although that's hard to prevent completely; a landscape format is more prone to this than portrait format. This would be helpful for you to judge oh much of a white gutter you want to accommodate.
It didn't sound like you are doing any crossovers, but if you were on a perfect-bound job, I would repeat a bit of the image in the center, if that makes any sense.
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