What is the impact of InputAssistant on the SAP system performance?  

GuiXT runs on the the client (frontend) PC. There is no performance impact on the application server or on the database. We measured around 5-10 milliseconds for a typical screen and with a script of medium length, on a 266 MHtz Pentium PC. The user will probably not realize any performance impact. If you store the scripts on a central server in your LAN, or on a webserver, this causes increased net traffic. It might make sense to use the built-in local cache of GuiXT if several hundred or more GuiXT users are expected.
If you generate the "local value help" of InputAssistant, then you can save a lot of F4 requests to SAP system (less application server and database load).