Okay, so perhaps I have an apology to make to vSphere Replication 
The networking chaps had made a bit of a mistake in the QoS being applied to the link and it was throttling all traffic to 5Mb/s. Now this has been fixed, things are buzzing along much better. We've been given full reign of the bandwidth for the moment (as it's the weekend) and everything has caught up nicely. We've even copied a few of the smaller VMs in their entirety rather than taking a initial copy over to the DR site first.
I'd be interested to know if we can safely double the extents and buffers in 5.0 environments to match the values used in 5.1 environments. I've got another SRM / vSphere Replication job coming up and I'm not sure if the customer is on 5.0 or 5.1 so could be really useful information.
I'm sure the original poster (whose post I've kind of hijacked - apologies!) would be interested as well 
Dave