Based upon what I've seen and experienced - - -
Many of the "brownout" problems are a result of poor connections, 22GA wire, servos with high peak currents, Battery packs wih high internal resistance, and so forth.
Step one can be to add a 540uf capacitor between the battery leads, close to the RX.
Step two is to use any one of several schemes that route the servo current around the RX.
Step 3 is to use a SBEC capable of the needed current. It's possible to use multiple SBECs when the number of servos makes it reasonable.