We have 4 extenders and 8 tuners (CetonInfiniTV4 and 4X Magyca ClearQAM)
Not sure what you mean by "less powerful" HTPC does not need to be "powerful" to begin with. Without the extenders, or with just one extender, Intel ATOM Dual core was more than enough to drive live TV from 2x ATI DCT's.
One thing to keep in mind is that Microsoft recommends 1 CPU core and 1 Gb of RAM per extender connected.
If you have TV connected to the HTPC, add another core for it, as well.
With 4 extenders and a TV connected to HTPC, I can run all 5 TVs with 8 Gb of RAM without a hitch.
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!!!