The lifter and valvetrain mass IS the determining factor to RPM, that and rotating assembly weight and balance. A flat tappet is good for higher RPM, but if it's a hydrolic, then it can limit the RPM. A solid lifter basically will not limit your RPM at all. Solid is the way to go for a race car, not much for a street car. You have to run a lot of valve spring pressure which can tear apart valvetrain parts. You can get 6500-7000, or more, out of a flat tappet lifter with the right combo.
Nick