Sadly, it's pretty much the upper limit of lockstep networking. Until we go all IPv6 and get global fiber in this country, there's just too many damn hops and NAT that introduces their own extra time to the packet round trips. Add in the minimum 65ms latency on the 360 controller.
They could go asynch and remove the input latency completely, but they run into two issues:
1) Each AI entity now incurs it's own bandwidth cost. Enemy counts would go down, and I know some people feel Reach already doesn't have enough enemies in at one time.
2) Even if the developer went "fuck it, do it anyway", Microsoft (and Sony) have hard limitations on how much bandwidth a game can use at any one time. Know why games like have less players on 360 and PS3? It's because those extra players go over the bandwidth limit. Dedicated servers fall under this limit too, so putting Firefight on dedis wouldn't help. I believe MS's limits are in effect so that you are supposed to be able to play Live on as low as 256kb/256kb broadband.
The upside to lockstep networking is you can basically have infinite amounts of AI with no extra bandwidth cost, since all that's being networked is button presses. You're only limited by RAM. Reach's theoretical 40 max AI and a future Halo title with ~500 enemies in a battle would take the same bandwidth.
The upside to asynch is that you could do drop in/drop out campaign and Firefight.
I understand the concepts behind lockstep networking. I am not convinced that Reach's implementation is the pinnacle of it, and it will never ever get better, forever and ever amen.