performance impact of growing rules files

[Ramprasad]

Hi,
  We scan mails for quiet a large number of domains ( around 1.5k
domains). The scanning happens on multiple identically configured MS
+postfix+SA  linux boxes behind load balancers

   For every domain that is added there will be entries in
spamcheck.rules spamaction.rules etc. Besides the domains will have
their own whitelists and blacklists which go into whitelist/blacklist
rules files. Already these have more than 10000 lines  each

I am not sure how this architecture will scale. Additional hardware is
not a problem , but the solution must scale

 Assume I have 10x more domains and traffic next year .. will there be a
performance hit because Mailscanner has to read such huge rules files.
What will be a 100% scalable architecture 

Thanks
Ram