Its really hard to tell 15% apart
Not really. For the 45 g switch that's almost 7 g, and as you can compared the switches
in situ, it wouldn't be hard to notice a difference. Also, in the worst case that could be ~38 g switch vs. ~52 g on the same keyboard, which would be very easy to differentiate between (as you can easily feel the weight differences on the variable keyboard).
I think that the +/-15 g figure is a misunderstanding, as it would make little sense to build separate 30 and 45 g switches, if the tolerances were so high: 30 g + 15 g extra would make it the 45 g switch, and vice versa. Simple QC at the factory for the domes could sort those easily. I also think that the +/-15 % figure is some worst-case scenario. However, the exact info on the Topre switch tolerances doesn't seem to be easy find.