rs-232 theoretical maximum speed
The maximum possible speed depends on the capacitance of the cable and the impedance of the source driving said cable. There are also some tricks that can be done at the receiver end to affect the maximum achievable bandwidth, though you are eventually limited by the communication channel, as described in the Shannon-Hartley theorem.

As such, the voltage range the driver can produce is largely irrelevant, assuming that you have a receiver that will work with whatever voltage range your transmitter supports.

You are incorrect in your belief that the voltage plays a role in determining the maximum speed.



Furthermore, the "speed-vs-cable length" tables are what's called a rule of thumb, e.g. a general rule that works in most cases. They generally describe a safe starting point, though in many situations, the system can actually go faster.