The voice option may use more current and drain the tx battery so going smaller will only require more frequent charging. Not a big deal since just a few years ago we usually charged before each time we went flying.
There is another option for the voice electronics, use the Hitec method and hang it outside I'm a box.
Maybe rewire the Futaba trainer connector, if it has enough pins, to carry all the required data and power for the add on module. In fact, it could have been prewired for that and HK would have had a new option accessory to sell.
Works for Hitec.