It is very possible to uncap, and its not a very hard process. It really all depends on which cable modem you use, and what firmware version it has. However doing this, is ILLEGAL, no you will not get arrested, you will however have to pay a small fine, and could result in your account termination.
First off, a 1.5 megabits per sec does not really mean both and and down. Now first off you are limited to the speed on the other end, mabey its capped half the time. Finaly 50 - 120kbytes is about right, at least the 120. I mean, technically you should get about 180, but the point is, its not that far off.