Interesting that they say "As we suspected, as the battery SOC drops the power drops off significantly, far more than the RWD cars. This is because the voltage sag is significantly greater with the much larger amount of current being consumed by both motors."
I was thinking that I must be insensitive to the major power changes I had read about but it seems the SR+ (which I have) has less variation in it's power depending on state of charge. Of course it has less power to begin with so there's less to lose ... so it's just as well!