output resistance of current mirror
The one way of finding the\$R_{OUT}\$ resistance in a proper way is shown here: 





And what you should know that the LTspice first computes the dc operating point and then based on this result (gm, ro) the AC analysis is performed.

All this means that if you include \$\lambda\$ in your MOSFET model. 
You no longer can assume that \$I_{D1} = I_{D2}\$ and \$r_{o1} = r_{o2}\$

Because the lambda will also affect the DC-operating point also. 

And the equation for \$R_{OUT}\$ is \$R_{OUT} = ro2*(1+gmR_S)+R_S  \$

I/O Resistance of common source MOSFET with source degeneration