By how much should I shorten the cooking time of a slow cooker recipe if I'm only using half the meat?
You should probably not reduce the cooking time by a lot, if at all. These recipes assume you are keeping the meet submerged in liquid at some constant temperature for several hours. Assuming your tenderloin is not somehow less thick than a 2 pound cut would be, nothing in the recipe changes. The core temperature of the cut won't take any less time to get up to temperature, and the lower thermal load from having less product will also not affect slow cooking.
Ideally, have some flexibility in when you expect to be done cooking and stop when you have reached the desired results, rather than after some arbitrary period of time.
Apart from that: if you are having problems with meats coming out dry, you might want to check out this question and answers. To summarize: cooking lean meat for long times is usually not ideal.