Get rid of the 2 additional HTPCs that have statically assigned tuners, and use Extenders/XBOX. You can have up to 5 extenders on at the same time, but you can add as many to the system as you want, you will just be limited to 5 on at the same time.
Also, if a lot of the programming you view is on broadcast TV, add cheap ATSC, or Clear QAM tuners , and assign those to unencrypted channels, thereby preserving the Ceton tuners for channels that need the cable card decryption.
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!