Not sure what size boat you're running, but I'm 6'4" and the 4 bay DRE frame on my 16 footer has a cockpit that's plenty big. I can even sling a ammo can on either side - makes it more cramped, but the length to the footbar is still fine.
I'm a fan of the diamond plate for the pure reason of getting heavy cargo (cooler, kitchen box) in and out. The slip resistance is great, and the way I instructed DRE to weld them on (cutouts) gives all my cross-pieces about 5"+ of adjustment. Seems to be plenty of play, given that my kitchen box isn't likely to ever change dimensions & one of the other bays is dedicated to a cooler. Most of the major cooler manufacturers
range between 18" to 22" in width for the 100 to 200 quart sizes, and anything smaller can be secured in a drop bag.
The frame is heavy, but still manageable for me to put it on top of my truck by myself. If you have a 15'+ boat, I would think that the 4 bay just gives you better options. I have 2 plywood pieces that cover the 2 bays in front of the cockpit on day trips- cover them in pacos and it's a perfect lounge deck (with the day cooler in a drop bag under my rowing chair). Now if you're using your cooler or drybox as a dedicated rowing seat, then maybe that screws up your configuration flexibility. I can't go 3 bay, but I also can't think of a good reason that I would want to - the weight isn't that much of a factor on a 16 footer IMO. Plus I usually trailer it inflated anyway.
I did the attached PDF sketch a few months ago and it doesn't include the diamond plate cutout dimensions, but generally the crossbars have about an 8" cutout width along the rail. I can measure that when I get home tonight. Also - make sure you get them to cut out the plate adjacent to where your D rings are on the boat, plus the corners. I also opted for all the cross pieces to be pinned, so I can break it down somewhat if necessary....although I don't plan on it.
Hope this helps.