Given all of the above, and knowing that my MM input is configurable for inputs from 1mV to 15mV, is there any benefit to using the lower 1:8 ratio instead of the 1:16 ratio?
Also, do I need to worry about capacitive loading, like with a "true" MM cart, or does that not apply to stepped up MCs?
In order:
There may be, but without doing some scope testing, you won't know. The step up calculations don't take frequency response into account (that's a far more complex situation) and that's where one configuration might be better than the other. As I've said over the years like a broken record (!), the details of loading the transformer secondary are extremely important and are not attended to in 99% of setups, despite involving about ten cents worth of components and two minutes of soldering. IMO, anyone charging more than a couple hundred bucks for a transformer box ought to do this for the customer.
The loading of low output MC cartridges is, despite legend, extremely non-critical: unlike MMs, their inductance is negligible. Loading of the transformer secondary (when used) is the key issue. The only things the cartridge load resistance affects is level and (to a small degree) noise.