The M12 Redlithium batteries are pretty amazing, and these 4.0Ah XC batteries seem to pack an extra punch in addition to their longer run time. They charge very quickly too.
I had been using the standard 1.5Ah compact batteries with my M12 tool lineup, and finally got a hold of one of these 4.0Ah extra capacity batteries. The big boost in run time was the obvious bonus, but since I've heard that the XC batteries can supply certain Milwaukee tools with more power, I thought I'd run some experiments to verify the speculation. The idea makes sense in theory: more cells connected in parallel allow for more current to be discharged without the tool overheating as quickly. More current draw equals more power.
The first test I ran was with my M12 5-3/8" circular saw. I took a sheet of 1/2" MDF, and purposefully wavered my cut line while I was ripping it. I stalled the tool while using the M12 compact battery, but only bogged it down while using the M12 extra capacity battery. Keep in mind that this was a very unscientific test, since I had no way of measuring the amount of torque I was applying to the handle in my attempts to get the blade to bind.
The second test I ran was with my M12 1/4" FUEL impact driver. I drove a 1/4"x4" lag bolts into the end grain of a 2x4 and timed how long it took each run with the different batteries. I attempted to just let the weight of the tool drive the lag bolts, rather than pushing down on it, since I figured it would be more consistent. The XC batteries are around twice the weight of the CT ones, so that may have played into the driving speeds a minimal amount. I repeated the test four times for each battery type, and ended up with an average of 4.3 seconds for the XC battery, and 5.2 seconds for the CT battery.
So both my "tests" came out in favor of the conclusion that the XC batteries supply a bit more power to the M12 tools (at least the FUEL versions). My experimental results are backed up by referencing maximum torque specifications listed on XC kits vs CT kits for the otherwise same product number tools. The extra capacity kits generally list 8-12% more torque than the compact ones. Examples include the M12 2403 drill/driver, and the M18 2604 impact hammer. Note that both of those are FUEL models, so that may play into which tools utilize the extra current output capacity.
Durable, Sturdy, Good Size & Weight, Long-Lasting, Powerful