widgeon,
You seem to think that I say 4 blades is "better" than 5 blades, which is not true, at all! The 5 blades are more expensive, but could be a better mix when all design factors are considered. Obviously, Sikorsky thinks so,too. I have hundreds of hours in a 7 bladed monster!
semirigid,
You hit the nail on the head! Increasing rotor rpm is a way to add extra equivilent blade area without adding any blages. In effect, the rpm makes up for the lesser blade area, so that thrust is increased without any rotor head/blade change (as long as the CF can be tolerated.)
The peak thrust of a rotor is basically determined by the Ct/sigma (coefficient of thrust/solidity), where all rotors are limited to a set value of max Ct/Sigma (usually about 0.20). Ct is thrust/ [(air density) x (disk area0 x (tip speed squared)] and solidity is [(number of blades) x (chord) x (radius)]/ rotor disk area.
Sounds complicated, but it is basically just like an airplane wing, where the wing stalls at a fixed coefficient of lift, so you need either more wing area (rotor solidity), or more forward speed (rotor tip speed).