Discones have 0dB gain. How much additional signal you may get by putting an antenna outside in an elevated location depends on the materials in your building, your indoor antenna, the coax you use, the topography in your area, and various other factors.
If you're on top of a mountain in a small wooden cabin, an outdoor antenna won't help all that much; you'd have a fairly clear line of sight either way, and the outdoor antenna may only get you an additional 3-5dB of signal.
But if you're in a building with aluminum siding in a small valley, the outdoor antenna may give you an additional 30dB of signal, if it offers a clear line of sight to the transmitter.
You'll probably find yourself between these extremes.
Sorry, I did not word that properly. I understand that a discone has zero gain and that a yagi / directional adds positive gain that is in a specified direction. I was curious if using an outdoor discone would increase the SENSITIVITY of the CC mode. ?? And about how much? I used the DB as an example.
Upman is correct that a outdoor antenna will boost a medium-strength signal competing with the signal you're trying to get a CC hit on by roughly the same amount.
BUT an outdoor antenna will help because it will raise all signal levels higher above the noise floor of the receiver. I occasionally get CC hits from Virginia in Pennsylvania with a 436 + outdoor ST-2, which I do not ever get with a HT whip/duck antenna. But predicting the exact amount of difference is nearly impossible, for the reasons I mentioned earlier.