I have thought about going to nutserts and plugging the holes. Knowing what I know now I would have installed them that way. Floor is way too thin to provide any meaningful support even with their backing plates for the floor bolts.From your original photos the backer plate looks to have an equal reveal between the door opening and top of backer plate. In the latest photo the reveal is wider toward the front versus back.
From JCR's video installit appears the backer plate is held up in place and drilled. Then the slider is bolted into the clips on backers tabs, followed by "no preloading" and drilling up through the torque box and floor. *Cringe* (Nutserts all the way!)
If you bolt it up using the existing holes and the backer plate is flush across the entire piece without the slider on, does that change only after bolting the slider on?
It may be that one of the tabs on the backer could be welded on a touch high which could pull that area of the backer away from body creating your gap.
I have to say that the Savvy Rockers/Sliders I have and installed are in a different league than those. Worth every penny!
I checked every single tab on the backer plate to check just that. They are all the same measurement and the backing plate is flat not bowed or distorted. It's flush until you bolt up the slider which then preload the backing plate pulling it away from the tub where it has less bolts. The front most bolt.on the fender does very little to keep the backing plate tight against the body. That sheet metal is just too thin.
If savvy had any in stock these would go on marketplace tomorrow