Dust will affect the radiators of AiO Water Cooling solutions as well. (Like 4Ryan6, dust is a fact of life.)
Discussing the Silver Arrow is interesting, since there is a lot of debate, particularly about it and the NH-D14. It amounts to a lot of splitting hairs, and a lot of this is muddled since there are now two types of Silver Arrows. Long story short, your Silver Arrow is fine, and you'll be hard-pressed to get something with superior performance. It certainly won't be worth dropping at least another $80 for minimal gains.
As for motherboard warping and stress, that is another subject on its own. As far as I know, no objective, conclusive tests have been done on the subject. I do believe, with every use, the motherboard, expands, shifts, and bends - as it should - because of the heat. The mounting kit adds another source of pressure. Some mounting kits allow you to overtighten way too easy (and, even if they don't, some boards are just that flimsy) and that is probably where most of the cases of damage caused by a heatsink come in. I am not saying one should ignore the extra stress a heavy heatsink can add on, but it is easy to forgot we throw other heavy stuff on our motherboards as well - GPUs can weigh in excess of a kilogram as well. Even though the motherboard isn't taking the full weight, it's still a point to bring up.