Lets say you have a 500Mbps connection (most have higher). You're already sharing that with 9 B41 carriers (3 carriers / 3 sectors). Up that to 6 carriers/sector and you've got 18 B41 carriers to support. Assuming equal load balancing (1 connection per carrier), that reduces your possible throughput to ~27Mbps per carrier. To be fully effective, each carrier should have full support from the backhaul, which would require a roughly 1.5Gbps connection. And that's not considering the B25/26 and 1x/EVDO connections.
No, backhaul equal to 100 percent of maximum airlink capacity for multiple carriers across multiple sectors is not needed "to be fully effective." All carriers and all sectors running at full bore simultaneously never happens. Never, not even close. Provisioning backhaul at the 50 percent level for aggregate airlink capacity should be more than adequate.