Signal strength does not really matter unless your signal is breaking up, you either get the channel or you don't in the digital world. Changing the output of your box from 1080i to 720p is not going to get rid of the pixilation, it is a problem at the source. NFL HD is broadcast in 1080i so theoretically setting the box to 1080i would be best as you are not doing any more conversions on the signal. But if you have a 720p display it has to get converted to that resolution at some point and then it is a matter of whether the box does it better or you TV.
This week was definately better than last week and if they can make another step improvement like this they will be on par with CBS and at the top of the list for PQ. Until then I consider it watchable but not nearly what HD should be.