Questions concerning dual flush converters
We put a dual flush converter on my parents toilet a year ago.  It was pretty crappy (pun intended).  The problem we found, was that the half flush didn't always clear everything out, and much of the time you had to double flush the half flush.  When we called the manufacturer, they stated that with some old toilets, the converters don't work so well.  Luckily HD was willing to take the kit back, so we returned it after a couple of weeks of trying to get it to work.

Granted, this was one of the first dual flush conversion kits, and they may be better by now, but YMMV, as it seems to depend both on the kit you get, and the toilet it is going on.

As for special considerations for septic- the less water you can use the better, so in theory converting to a dual flush should be of benefit.  After our experience with my parents, for our new house (on a septic system as well) I just went with a high efficiency 1.28GPF toilet.