We've put tens of thousands of acres of soybeans in the ground at 8 mph with various seeding tools and our stands have been fine, thank you.     We've done numerous trials increasing speed every pass...have done it with most tools we use and in conditions varying from fluffy to hard notill.  Provided you are using a tool which is capable of handling the conditions the speed is almost irrelevent.   The only exception has been in extremely loose fluffy silt loam with a drill with two ranks of openers, where you can get into situations where the rear rank throws dirt over the front rank....with a proper conventional drill (Great Plains Solid Stand) this started to occur as speeds got around 8-8.5 mph.  The CrustBuster Allplant with it's 3" pointed press wheels and lots of weight can cause it at somewhat slower speeds, say 6.5ish in extreme conditions. 

I'll bet we've put more beans in at 8mph then below that.