Your 7200s CPU utilization will depend on what services are running and the throughput of the traffic. Adding WCCP to what is already currently running will also add more overhead. I have seen a 7200 w/G2 processor have minimal impact (10-20% CPU) with over 100 branches. It really depends on the results of what you saw in your POC and how it effected the router.
A 7600, if configured correctly, will handle the redirection in hardware and you can also change the default load balancing MASK. So yes, it is a more powerful platform.
In either platform, make sure you check for known bugs in WCCP. :-)
Good luck!
Dan