Be at peace, child of automation.
There is a timeout value associated with this "Register Object" in the 20-COMM-E. Page C-6 of the User Manual describes it as the Register object's class attribute 100, the "Control Timeout". I think it's zero to start with, and needs to be set to a nonzero value before you can use this control method. That value is also accessible using the PCCC Object, as "integer register" N42:3 (see page C-16).
The real performance drawback is that the SLC-5/05 Ethernet port is served by the CPU itself, so if you have a long program scan time or the port is very busy serving HMI traffic, you can have a small delay in acting on your MSG instructions to the drive.
I have never taken the time to write a full application technote, including performance analysis, for this sort of application because that would be tacitly endorsing it, which I don't.
If you are very concerned about the I/O reliability of the Ethernet connection, you could just hardwire the Start/Stop connection and send the speed value by itself.