Actually, 32bit RIMMs are effectively 2 single-sided 16bit modules back to back on a new PCB. The latency would be no different than dual-channel 16bit. The latency is caused by the number of devices (chips) used and the individual latency of each device. The devices used in 32bit RIMMs are not currently 32bit. If Rambus ever releases 32bit devices they will probably yield lower or similar latency.
I thought a thought, but the thought I thought wasn't the thought I thought I had thought.