I'm considering purchasing an external mic pre/channel strip for use with
a Yamaha AW2816. My question: is there an advantage to purchasing
a preamp that can be equipped with A/D converters, such as the Focusrite
ISA220, as opposed to doing the conversion with the stock front end?

There's 3 issues:
- quality of the a/d converter (probably favors the Focusrite)
- RF environment (no RF-producing digital circuits in a preamp)
- power supply (HDR shares power between 'dirty' devices like motors, LCD
displays, other DSP)

Factor all that plus price into a cost-effectiveness equation and see if you
get what you pay for. Also you'll probably have to install the a/d card
yourself, and if it's anything like the Octopre, it's not entirely without
its challenges. No big deal if you've built a PC before, for example.