I answered my own question. In the SAPI 4 specification there are control tags which can be imbedded into the text string.
TTS of "Hello, this is..." would be read at the default speach rate of probably 150 words per minute. "\Spd=100\Hello, this is..." would be read at a rate of 100 words per minute. The range depends on the engine, but 50 to 250 is the spec.
This works in IVM, simply put the tag in the script.
\Pit=<num>\ changes the pitch \Vol=<num>\ is the Volume etc. etc. See the MS SAPI 4a SDK documentation, search for "control tags" for a full list.
Rodgers Moore