MI means you could be performance weight limited at the alternate. The 154,000 max landing weight limit is, in this hypothesis, a performance limit rather than structural as is the 148,000. Although it all depends on how much faffing around you do at the destination before you divert so probably would never be a problem.
You would have to burn
more than the trip fuel to avoid landing overweight at destination because you'd need to get rid of the contingency too
