The C rating is closer related to an air intake on newer cars (the intake itself, *not the filter*)
20C is a stock air intake
40C is a K&N, so although it has the potential to supply more air, the engine doesn't necessarily need it, but whatever air is being supplied is more available, so you're getting very small performance boosts.
As for the draw, I wouldn't worry about it if you know your shimming is good, and it's only a 320fps spring.
I'm curious to see people's battery efficiency using a 7.4v to run high speed gears around 1200-1300rpm mostly full auto, because I went to the opposite side of the balancing act; I'm using heavy weight triple torque gears and an 11.1v lipo to reduce constant and startup motor torque, increase the momentum on the gearset to more easily maintain full auto.
Here's an old thread, but very insightful motor info from minnesota airsoft
http://forum.mnairsoft.org/viewtopic.php?f=30&t=28181
My EF1300 stacks up pretty damn well to other motors, and keep in mind it's just a ferrous magnet motor. Enough torque to not bog down at any point and plenty of speed for full auto. Although a G&P M140 has a higher top speed, it's not nearly as efficient, it immediately smells of electrical fire after the first burst...
Soooo yeah, let's talk about motors O_O lol