The EFI on ATV's can't compensate for any mods they don't have a O2 sensor to read air/fuel ratios. If they could we would not need the fuel controllers that are being sold today.
None of the sensors in the EFI system know that more air is going in the engine when you add a exhaust, intake, cam, pistons etc.
You have the following:
Air intake temp sensor
air intake pressure sensor
crank position sensor
water temp sensor
Seems like I am forgetting one but the point is none of those can tell you added a exhaust. Cars and trucks have a O2 Sensor that reads air to fuel ratio and they can compensate for it somewhat. They are called a closed loop system.
The answer to your question is no controller needed, a slip on will not change much in the way fueling requirements it also won't change much in the way of power either. The power is made in the pipes not the cans.