what is Siri consuming? is Wifi enough? or 3g celluar data is involved while Siri searches online?
You will save on data usage if you only use Siri at home with a wifi connection. Siri sends the raw audio of your talking to its servers, so the data usage will be comparable to using VoIP. Although that's only if you talk constantly to Siri (which I'm actually seeing some people at work doing these days).