Big companies are all the same
It's sometimes hard to get a tech to help you because they deal with a lot of people every day and they make assumptions that could be wrong. cOre is an example of good techs and if you keep trying you will get one. Most of the time I find that if I mail them a trace route to a server then someone can see EXACTLY what node is dropping the packets, it might not be them but some back bone provider. Cable systems are complicated because they use RF that can leak as components degrade and everything has to be tweaked and balanced just right. And they really were not designed to upload traffic, it was a one way system. I haven't been exactly overjoyed with my own cable company but persistence pays off, and being nice when possible. Some techs are even eager to help because they love solving problems. A lot of set top boxes have a test mode that can check some parts of the cable. Last time I had a problem it was in the last 4 feet coming into my house. Oh and one more note, I know a lot of people like splitters, but man they drop the signal by 3dbm. That's half! For every one you use! Put the splitters after the modem, I think the TVs can use a lot weaker signal than the modems can. At least a tech told me that one day…
Oh and as far as uncapping a modem? It's not legal and that’s UPLOADING anyway. Cable should not be that slow, it's hardware or routing problems someplace.