output power in Tom model

I am getting s21 parameter around 12dB but I am getting output power -16dB for input power -20dB. I want to get 10dB of output power for -20dB of input power dB. Which parameter should I change in my mesfet TOM model to get the required output power.

If you want to see 10dBm output power while the input is driven with -20dBm, the Gp must be 30dB.
Changing the model parameters in a large signal model of a transistor is not solution because model has been extracted for that transistor and it must be constant during design process.
What is your intention by changing model parameters ? It doesn't make sense at all.Instead, you should correct your design to obtain desired performance.
If you amplifier has a 12dB small signal gain and if you get -16dBm output power when you apply -20dbm input power, there is a strictly an design error or an simulation error or something else..
If you post your design, we can say something concrete.Drawing is more explanatory than speaking..