[c-nsp] ISG: redirecting customers DNS to internal DNS Cache servers (L4 redirect)

[LM]

According with Cisco 
(http://cisco.biz/en/US/docs/routers/10000/10008/configuration/guides/ancp/isbl4rdt.html#wp1043711) 
I create the next configuration:

!----------------------------------------------------------------------------------------------------------
ip access-list extended DNS-Traffic
  10 permit tcp any eq 53 any eq 53
  20 permit udp any eq 53 any eq 53
  30 permit tcp any gt 1024 any eq 53
  40 permit udp any gt 1024 any eq 53

service-policy type control DNS-redirection

policy-map type control DNS-redirection
  class type control event session-start
   1 service-policy type service name DNS-redirection-profile

policy-map type service DNS-redirection-profile
  class type traffic DNS-Traffic
   ! maybe could be possible to use "match protocol dns" pending to test it
   match access-group DNS-Traffic
   redirect to group DNS-servers

redirect server-group DNS-servers
  ! are they going to do load balance? is it possible?
  server ip 10.53.0.235 port 53
  server ip 10.53.0.236 port 53
!----------------------------------------------------------------------------------------------------------

Questions:
- should I do modify the default timers (frequency and concurrent sessions)
- how is considered the value "sessions"? by ip? by request?
- what about the CPU? too much impact? any experience?

Alternatives with less impact to redirect DNS traffic? :]

Thanks in advance to all the list.
Luis