two things:

1. At lower frequency tank Q depends on inductor Q, now frequency decreases from 5 to 4.5, so you can expect reduction in the swing (but i am doubt of large reduction in the swing).

2. One thing i would like to point out is never-ever model cml buffer with only capacitive, you have resistive component also due to cgd leakage. If this is the case it adds huge loss to tank.  So please simulate buffer input impedance.

If i were you, i will start design including buffer.

Thanks,
raj.