How do you test the consistency models and race conditions on Google App Engine / NDB?
I am answering this over a year since it was asked.
The only way to test these sorts of things is by deploying an app on GAE. What I sometimes do when I run across these challenges is to just "whip up" a quick application that is tailor made to just test the scenario under consideration. And then, as you put it, you just have to 'script' the doing of stuff using some combination of tasks, cron, and client side curl type operations.
The particular tradeoff in the original question is write throughput versus consistency. This is actually pretty straightforward once you get the hang of it. A strongly consistent query requires that the entities are in the same entity group. And, at the same time, there is the constraint that a given entity group may only have approximately 1 write per second. 
So, you have to look at your needs / usage pattern to figure out if you can use an entity group.
Setup: Python, NDB, the GAE datastore. I'm trying to make sure i understand some of the constraints around my data model and its tradeoffs for consistency and max-write-rate. Is there any way to test/benchmark this on staging or on my dev box, or should I just bite the bullet, push to prod on a shadow site of sorts, and write a bunch of scripts?