The fact you have dedicated switches for IP Storage and vMotion in my opinion makes Jumbo Frames more attractive, so that's one tick in the box.
Regarding your backups, by the sounds of it they are still agent based and therefor dependent on the network? This is something you should address as agent based backups (as you probably are aware) are a high impact to compute/network and storage and eliminating this overhead from your environment has numerous advantages.
For your W2k8 VMs, i would suggest upgrading them to VMXNET3 virtual nics as this will improve performance (where high transaction or throughput is required) and will allow you to custom set the MTU.
Here is an article explaining the various Virtual nic types.
However, if the processing is done on the VM and it does not have a dependency outside the VM itself, then I wouldn't expect the network performance for the VM (excluding for agent based backups) to improve with the VMXNET3 or Jumbo Frames as network traffic shouldn't be your bottleneck.
At the environment stands right now, I would investigate and confirm your not suffering any fragmentation, and if you are, lower the MTU until you can upgrade your switches.
You should also ensure the switches you buy meet your requirements and to do this, you should document your desired end state, and work backwards to ensure the new solution will achieve those goals. You also need to consider the fact your using 1GB, and how much more benefit you require from this upgrade. While its obviously more costly, moving to 10GB especially when using IP Storage (even when left on MTU1500) will give you a huge performance increase assuming your storage has 10GB ports.