Drop stem on endurance bike to get the geometry closer to road performance? So I'm planning to get this bike, possibly the later 2017/2018 model:

http://www.polygonbikes.com/id/bikes/description/2016-strattos-s3-black#spec

Which is a well known local brand and practically any bike shop near here should carry one.

Anyway, the bike has more of an endurance geometry, though I wanted a more aggressive one, they don't have one at this price range.

The question is, can I simply swap the stem to drop stem (negative rise) and move the stem down the spacers to get to a 'performance bike' position? or will it still be a different geometry altogether? I can't see much difference in the final position vs an actual performance bike, but I might have missed something.

If it makes a difference, I will be using this bike mainly for 'commuting' to work (I usually ride hard on threshold anyway), some climbing practice, and maybe a few amateur races just for fun.
Yes, you can get a more aggressive riding position by dropping the stem down on the steerer tube, and swapping the stem for one with a larger angle (and possibly a different length). 

Bikes typically come with 3 5mm spacers, and swapping from a 6 to 12 degree 100mm stem drops an additional 10mm, so you should be able to easily drop the bars by 35mm.

What we cannot tell you is whether you will be able to get the bars low enough for you. I suggest that you use you current bike as a baseline if it is set up the way you like it. Measure its stack and stem rise, then compare that to the bike you are looking at. Also, go get a test ride if that is possible in your location, or at least sit on the bike and get a feel for the rider position.

There are other differences in steering geometry between the bike you are looking at and the higher end ones. The higher end ones have slightly steeper head tube angles and will have slightly quicker steering. Again, you will have to determine if that works for you.