Should I upgrade MBP15" late 2011 HD cable to 2012 cable?

Almost correct! The data width is the same across SATA I, II & III interfaces, So thats not the issue.

The issue is twofold: The first is the still older MacBook Pro's had a SATA II cable design and was not able to support the higher data rate of a SATA III device (SSD's more so). In your case unless someone put in the older cable you don't need to worry about that.

The second issue is the cable tends to wear from rubbing against the rough surfaces of the uppercase as well as the bends tend to fatigue so the cable may not be up to the task of supporting your new drive.

So... The long and short of it is I always recommend replacing the cable when installing a new drive just to remove the possibility. I should point out a weakened cable can exhibit odd behavior and can in fact corrupt a drive to the point it can go bad!

I do recommend putting in the 2012 cable as its a better designed cable, in addition I place electricians tape on the uppercase where the cable rests to help prevent it from rubbing on the rough aluminum.

Lastly, there are two plastic clips on the mid-plane which clip into the bottom cover. Often these get broken in the 15" model. I always place a working clip near the optical drive as it helps protect the bottom cover from smashing into the HD SATA cable and I add foam tape on the optical drive to help protect the cable as it goes across creating a protective channel.