Ok, so it looks like they are the same afterall...
I got the chance to look at and weigh both the M11"B5 and the B5 premium in close succession. Construction seemed identical, flex felt the same as far as I could tell. According to high-precision
digital food scale I got at Target a few months ago, their weights per pair were as follows:
2006/2007 M11B5 158cm 8 lbs 1/2 oz
2006/2007 B5 Premium 158cm 8 lbs 2 oz
That is to say, they weigh the same. Looks like that Magnesium stuff is just a big confused lie. Incidentally, the B5 Premium I was looking at was a pair that for some I was able to buy brand new on ebay, with NEOX 412 ALU bindings, for all of $385!
While I was at it and because I like to measure stuff,
2007/2008 B5 162cm 9 lbs 15 3/8 oz
2007/2008 M9 164cm 7 lbs 8 1/4 oz
2007/2008 M9 157cm 7 lbs 12 7/8 oz
NEOX 412 ALU 6 lbs 8 oz