Took me a bit to realize what you meant by "spill deflector guards" Gavin. We call them side boards here. Yes, they cause a lot of extra stress to draft arms, hoist cylinders, goosenecks and tractor frames and mounts by increasing the load capacity quite a bit as you mention. This also increases overall scraper weight, which takes it's own toll on tire life, brakes, and lower fuel economy. Not to mention increased haul road maintenance, which has to be figured in.
Some folks figure it's worth the extra cost to move material faster, but I'm not so sure myself, having spent a lot of my career around scraper spreads in maintenance and repair. Newer Cats take the increased weight much better than the early models I started with. Back in those days we saw a lot of stuff busted by trying to load past the factory recommended level. Oh sure, you could get away with it for a while, but sooner or later the machine and your pocketbook will suffer.
One thing you can bet the farm on, is that if you install side boards to contain spills, the operators will just continue to load till the muck slides off no matter how high you build the dern things. That leads to slower cycle times and just ticks off the pushcat guy something fierce because he has to stay behind the scraper longer.
I've taken more side boards off than I ever put on as machines came in from other jobs. Often it was too late as my job got saddled with the higher maintenance cost of repairs that came with them. Were it up to me, I'd never put them on, but production people generally rule you know.
