The motor is only 3 amps, so it can't have much power.
For cutting bar and angle in my own shop I usually use a couple of old 7 1/4 circular saws with abrasive blades. It's an inexpensive solution and usually good enough.
It turns out that the 7" blade gives just enough surface speed out at the edge to do decent abrasive metal cutting, but the downside is that the saw motor has to have enough torque to keep that speed up, and although a hand-held circular wood saw does tend to have that much torque, it is also one big unwieldy thing to cut metal with. Certainly no good under a car, but extrapolating from what we know about using that saw in the shop to cut metal, there is a way we can get an idea of what kind of job a cutoff saw will do.
A typical 7 1/4 circle saw draws 12 to 13 amps. The max. a normal 110v. wall outlet is rated for is 15 amps.
Your question got me to thinking about how to compare the power of various saws. I know about this stuff because I designed motors for various companies for years. Motor technology is pretty mature and the same technology is available pretty much everywhere as far as power is concerned. There isn't anything real expensive in an electric motor, so unless you are at the really, really high or low end of the price range, there's no real benefit in making one that is more or less powerful. All 110V AC motors are basically copper wire wound on a laminated iron core. More copper windings make for more power, but the price of the copper windings is so low compared to the price of the motor that there's little reason to save cost there. Now it's very true that it costs money to beat heat, and so better motors handle heat much better. They may also have features like ball-bearings on the shafts, but the raw power of any motor in the short term is going to be pretty much dependent on how many amps it draws.
Since input voltage (110v) is always the same, what matters is torque. And useful torque out at the edge of the blade comes down to amps and blade diameter.
Manufacturers would have you believe that what matters is HP or horse power, but that just isn't true. Horsepower ratings are an advertising department's best friend. Horsepower is just a popular word for rpm times torque. The problem with comparisons based on high horsepower is that it takes torque to keep the speed up once you start to load the tool by actually doing something with it. But the adv. dept tends to rate the HP by the unloaded speed. Us people who buy and use tools know that only the speed under load counts....but that is almost never mentioned! Trust in this: As long as the rpm is high enough to do the cutting, more torque is what you want.
So one way to think about cutting anything with a circular blade is that doubling the amps also doubles the torque. It is true that doubling the blade diameter gives a longer lever arm and so is another way to give twice the torque AND it gives a side benefit by increasing surface speed which always a benefit in grinding. But there's a catch there. If you double the blade diameter, it requires 2x to 2.4x the amps in order to keep that blade turning at the same torque as the smaller one. So to use the advantages of a bigger blade, the bigger blade had better be turned by a motor drawing twice the amps. Otherwise nothing is gained.
Looking at it another way, if you have ever used a 7 1/4" circular saw with an abrasive blade to cut some steel in your shop then you probably have a mental picture of how that went. And that's handy because a smaller saw like a cutoff saw having a blade half that diameter would only need to pull half as many amps to have similar torque. So for a 3" cutoff saw, it would need to be pulling at least 6 to 7 amps to match the larger circle saw.
Hope this helps some...
Like anything, we can complicate it. If this gets anyone interested in design decisions and motors, or questions about stuff like that, maybe we should start a tool thread on motor design.
rScotty