The way I see it, Stormlight's "boost" is simply the fact that it instantly heals your muscles, meaning you never get tired. It doesn't vastly improve your strength, and Shallan wasn't able to run faster than Vathah and the others, simply longer.
With Atium, it is still basically impossible to hit the Mistborn. Yes, a fifth Ideal Radiant with a ton of Stormlight would be nigh impossible to kill with conventional means, even for a Mistborn with Atium. The Mistborn still has a few options, such as duraluminum-steel pushing a large chunk of metal at the Radiant to break their armor, or Duraluminum-pewter punching them in the chest, then using chromium to drain their Stormlight and keep them from summoning their Shardblade. On the whole, however, I think that the more combat-based orders would have an advantage over the Mistborn. Unless you give the Mistborn aluminum daggers. Those things can block a Shardblade, and any wounds inflicted by them cannot be healed until they are removed. All it takes is a little Atium, a close quarters throw or stab, and the Radiant is dead. Unless they use my patented idea: Shardglasses. (That would still leave the Radiant more open to having their Shardplate broken, and would limit their offensive capabilities, but it might keep them alive long enough for the Mistborn's Atium to run out.)