I believe I read somewhere that changing the slot cycle index on the phone itself will not actually change anything, except that it may drain your battery faster. As I understand it, the tower's setting for that attribute will override end-user devices' settings.
FWIW, I changed it on my Hero, and it made exactly zero difference in the ring delay (timed in seconds). If you watch a timestamped kernel debug log when a call comes in, you can see that it simply takes the phone a long time to get its sh*t together and start ringing. This is common across HTC phones. Piss-poor design for a phone if you ask me.