Honestly I don't have enough data to know what's going on. One of the pro calibrators can probably give you an answer as they have probably calibrated dozens of Panasonic plasmas.
All i know is Chad and Cnet measured the ST50/GT50 ~0.005, as did D-Nice until the 268hr (?) mark where it then dropped to 0.0028. I believe others have said they've noticed a similar drop on the 2011 models. Chad, D-Nice and Cnet all reviewed a smaller panel (50/55). Every 60"+ 2012 panel I've seen measured ~0.0022 - 0.003fL out of the box.
So it's possible that your MLL will drop after a few hundred hours. The ST60's filter could possibly give it a slightly lower MLL over the S60 too, like the VT60/ZT60's filter does over the ST60, but nothing significant... maybe 0.0005 at best.
So, in a nutshell you saying the 50" S60 and 50" ST60 should have the same MLL give or take 0.0005 fL at most? Also, around the 250-300 hr mark is when to expect the drop if it will occur at all?