The kit ones probably have ceramic magnets and so are pretty strong.

I'm halfway through the process of swapping out the ceramic magnets in a SD-made P90 from a '93 Hamer and replacing them with much weaker alnico ones from a Gibson P100 pickup I had lying around. Also doing it as I'd already replaced the neck pickup for a much nicer and cleaner sounding Iron Gear alnico P90 pickup, and it's out of phase with the bridge, so it gives me a chance to swap the polarity of the bridge pickup round so it's not really honky in the mid position. The bridge pickup also sounds too muddy when clean (it's as powerful in output as a humbucker) which is now highlighted by the new neck pickup, so a change to a less powerful magnet seemed the right thing to do. As I only just changed the strings when fitting the new pickup, I'm holding off taking the strings off again to change the magnets until they are a bit older and I can justify a new set.