One of the issues with SDM SOAP is that we do not support replication of our authentication tokens (called SIDs). A an SID token generated on a specific SDM server is valid only on that server.
So, when you try to use any web services integration coming into SDM over a Load balanced set of SDM servers, as long as you have stringent session Stickiness, then you maybe OK.
The main issue with something like UIM (or Spectrum or Process Automation Manager) load is that, they're usually initiated from just a handful of servers. (mostly one or two servers). So source IP based persistence will still create an un-balanced traffic because, your request would be sent to just (by F5 to) one server or the other on SDM side.
The above is something to keep in mind. We did have some customers who submitted some ideas about full load balancing support SDM SOAP traffic Balance of WebEngines/Domsrvrs for WebServices Applications (SOAP/REST)
Some had got around it by enabling JVMRoute based load balancing on AXIS How does CA Service Desk do load balancing and how can sticky sessions be enabled?
Hope the above two help you a bit more.
_R