Okay its been a few weeks now and I thought I'd take a look at the performance and how spiky it was. The highest hit element was the gadget with a few thousand hits so I took the data from the "default" image (-2,-1), (1,1) and looked at the performance over time.
Are all of the results over time. I'm not quite sure if that cliff is a code optimisation or is Google doing a step-fold increase but the noticeable element is that after the spike its pretty much level. Delving into the results further the question was whether there was a notable peak demand time on the platform which impacted the application.
The answer here looks like its all over the place on the hours, but the reality is that its the first few days that are the issue and in reality the performance is within a fairly impressive tolerance
Beyond the cliff therefore the standard deviation and indeed the entire range is remarkably constrained within about +/- 5% for min/max of average and two standard devs being +/- 2% of the average. These performances are the ones that within 1.2 times of the Google App Engine Quota Limit in terms of CPU so you wouldn't expect to see much throttling. The next question therefore is what happens under higher load...