4.3 is plenty fast enough to plant corn. We plant everything at 4.2. Don't ask why its not 4.3 or 4.1, I don't know.
The people that raise record crops, regarless of what kind it is usually plant between 3 to 3.5 mph. Slower speed equals more equal stand, more equal plant growth, ear size, everything regardless of planter type. Don't know anything about the singulators, but I do know a vacume planter is really nice.