Yes in Pega 8.3 you can set the throttling speed per SMS account level using the "Additional Settings" tab of the SMS configuration. You will have to enter a name / value pair type configuration there in the UI. The configuration of interest for you in this case would be throttleUseconds. Given its a pause in milli seconds between the messages, for your example you will have to set it with the value as 20 (i.e 1000 ms / 50 total messages per second). This setting also includes the split up of concatenated messages. So if you have 10 messages split into 5 concatenated messages. Then within that second it would have delivered 10 total messages.
The other factor / setting that would influence the delivery speed is the "Maximum Connections" in the first tab of the SMS configuration in the Pega. If you are having more than 1 as the value there. Then the rate would multiply based on that. For example with the above setting mentioned, if you have max connections set as 2 - then the SMSC will hit with a maximum of 100 messages per second not 50. Please take that into consideration as well.
A configurable pause between transmitting messages (micro seconds).
optional - defaults to 0 micro seconds.
The other related settings that might be of interest for you are :
A configurable pause between stopping transmitting messages upon receiving a throttle response and starting again (number of seconds).
optional - defaults to 5 seconds.
Throttle (pause as per value specified in flow control timer property) if a message queue full error is received from the SMSC (on=true)
default is false if not set
You can try few tests with these combinations of settings in the advanced tab of the SMS account in Pega.
Please provide feedback on what you find with these settings, it would help shape our backlog for future releases if need be.