The attorneys don't get to determine interest rates, they handle home registration only. The credit guys make the call on rates. They hand the deal over to legal risk who instructs the attorneys to deal with the matter.
All it means when they're on the panel, is that they're in a pool of attorneys to which deals get allocated to deal with. This is usually done in proportion to the firm's performance.
If a client requests a specific attorney on the panel get the deal, the banks are usually accommodating.
Remember, there are two attorneys involved here, the transferring attorney and the registration attorney. The latter works for you as buyer and are the ones that'll give you a discount.
The former works for the seller and they have zero need to provide discount to you.
When we purchased our current house a year ago, the transferring attorney wasn't willing to budge at all on fees, and they definitely weren't cheap.
The registration attorneys gave us a pretty good discount without us even asking.