If you say the ARs kill in 2 bullets, then I can say the UMP does too since you're using SP on the ARs for that effect.
I guess the only thing I'm trying to say is that although the UMP is very versatile and powerful, it's easily outclassed in many areas by other guns. A little elaboration is probably necessary though, just so I don't sound stupid.
-UMP without SP, which most people use, takes 3 bullets to kill up close. Even though it's an SMG, up close it can be terrible against someone with a Scar/Tar with SP if you don't have SP on it. If you put SP on the UMP, against the guns mentioned above, to me it just evens the playing field, making it no better than those guns.
-It's garbage at long range. It has no idle sway, but that doesn't mean it's good at long range. If I'm shooting you across wasteland with a Famas, I'll most likely kill you before you can shoot me more than once with a UMP. Same goes for medium range shots. Even though the UMP is good at medium range, I usually find myself losing those battles for various reasons.
It's a very situational gun if you ask me. It's good at close range assuming you get the first shot off, especially if you have SP, but at medium/long range against other guns I just can't seem to see how it can compare. Depends on the situation a lot, I know, but in terms of damage and accuracy, I'd rather have a gun like the Famas that is near perfect over medium/long range as opposed to a gun like the UMP that although can be used at all distances, is really only especially good at closer ranges.
I like to relate it to Mario Tennis 64. You can choose a character with power, speed, technique, or all around. If you choose a character with power, you're going to lack in some other areas, being the Famas. However if you use all around, being the UMP, you get the best of both worlds, but you're not specifically good in any area, just all around decent. In CoD, I'd rather have quick, accurate killing power, as opposed to having a UMP which is useful in many areas, but still lacking at the same time. If you were to have perfect aim with the Famas, you'd always beat someone that had perfect aim with the UMP.
It's so hard for me to explain, but this is the best I could do. XD Not to mention, tons of this stuff is situational, which I'm aware of. All personal opinion, but I think the Famas is one step above the UMP. Rant over.