I would agree with your attorney and start the SSD filing now. It takes a long time to get through the denials, appeals and hearings so the sooner you file the sooner you can get started. It took me over 3 years to get approved. I'm also PTD with W. Comp. I have DDD in 3 discs in my low back. I've never heard of straight leg raising being a condition of approval or denial for SSD.
I do know there are very specific criteria you have to meet, but this is just not one I've heard.
Two things that were very helpful in my case were a FCE (Functional Capacity Evaluation and a VE (Vocational Evaluation). If you haven't had both of these I would talk to your doctors and attorney about having them done.
Good luck with the process - if you have any questions, I'll be glad to try and answer them if I can.