Here are some of my recommendations.
- On both DCs enable the WCCP redirection on Cat6k.
- Enable redirection on Cat6k's WAN facing interface and Server's vlan ( interface to intercept LAN traffic) .
- Preferrably deploy WAAS in its own VLAN to avoid loop.
- Use Generic GRE (GGRE) as 'Egress method' to leverage h/w processing on Cat6k. With GGRE you need to use WCCP-GRE for redirection. I am assuming that you have either Sup32 or Sup720 on Cat6k.
- Use WCCP mask as 'assignment method' to leverage h/w processing on Cat6k.
Pl note that you can enable WCCP redirection on Cat4k as well. However, it doesn't support 'Redirect ACL', just in case if you don't want to redirect certain traffic. This is why I suggested you to use Cat6k as it gives you flexibility and yet achieve your goal.