The pacts have a minimum relationship required - and to answer the quesiton, I think I read that 10 is the highest you can get with "normal" means.  You can get up to 20 with envoys, and the pacts start hitting at 11.5 and go up all the way as they get better.

*Edit* don't forget that the envoy ship itself has some powerful abilities as well.  The Vasari envoy can double a planets production 1 minute of every 3, and you could have an envoy at every allied planet if you wanted to in addition to the pacts.