Okay, I got one of the spare 0.5 mm extruder heads mounted. I measured the output and discovered that I was getting swell to about 0.66 mm. That works out to 0.34 mm^2 cross sectional area.
When I was working with the 0.3 mm extruder head I was getting a swell to about 0.42 mm which works out to about 0.14 mm^2 or about 40% as much.
I tried printing a test raft using the old extruder set points and noticed that the base print roads for the raft, which had been well separated before, are overlapping now. In the morning I'll lop 60% off of the extruder's stepper speed and see what happens.