You thought about using its built in load balancing to scale out the capacity? Don't know what hardware you are using but it might be worth looking at the lease db and log file setups you are using if you can't add iron to the equation.
I'm looking over the documentation, and I see the -Y/-y options in dhcpd.
Maybe there is something worth considering rather than changing DHCP solutions.
(Other suggestions for DHCP replacement / scaling are always welcome)