The EXM file parsing (extension remapping) may still be in the MIU, however it hasn't been supported in some time.
there's no real practical limit on the number of routing rules from a performance standpoing - I've seen over 1,000 before. A large number of routing rules can be a real pain to administer via the web interface however - it's not designed for handling that many. But it wont break anything. Use a good naming convention for your rules up front - you'll be glad you did later.