Hi spirie, and welcome to the forums.
The general consensus is that ECI modems work better with ECI cabinets, although how much difference it makes is under debate. Some threads I've read would suggest they get less errors, and are therefore less likely to have error correction turned on, which will reduce the speed.
I did buy an unlocked one, and tried it, but it reported a lower attainable rate than the HG612, then my router threw a wobbly, so I haven't tried since.
It's pretty easy to tell which type cabinet you have, you can tell from the stats, but as you haven't got an unlocked modem you'll need to look at your cabinet, and
compare it to the ones here. By the way the PCP is separate to the FTTC cab
I'm on an ECI cab, and was supplied with an HG612 modem.