Seems to be the same issue occurring with Top Heavy as the one that still exists with Deadly Calm. As far as I can tell, after running tests on several guns with and without the mutator, it seems like the Top Heavy Weakspot bonus (for level 10) is triggering, but the damage boost consistently sits at around 7-8%.
Since this value is so close to the one that it's supposed to have at level 0, to me this indicates that (again, just as with DC) the upgrades are somehow failing to affect the damage boost values on these mutators.