Agreed. I've spent big money to get RF quiet inverters and SMPS's here, and I've sent more than a few back for a refund when they weren't as quiet as advertised !
I understand about the efficiency of these things too.........
Looking at the original post I'm wondering why 12.8 to 13.6 volts is a problem ? If the wiring is heavy enough and there is plenty of amperage getting to the radio and/or amp that voltage range should be fine.
If the goal is to eke out another few watts from the radio or amp it's a pointless exercise as those few extra watts aren't noticeable on the receive end. So I do wonder if there is really a need to get the voltage higher.