Had the same problem with my 2014 +4. I did drill into the floorboard to countersink the bolts fixing the seat runners but that didn't really solve the problem. Part of the reason that the bar under the runners bends - at least on mine - is that there's about 3-4mm difference centre to centre in the bolt holes between those in the chassis and those in the bar under the runners. Out with the file and extended the holes in the bars to match the chassis centre to centres, reattached to the runners, and that seems to fix the issue.
