My main question is if it is worth buying a soundcard for a stereo setup. I fail to see how there could be much improvement.

Interestingly, due to the directional tweeters in my speaker cabinets I sort of have a faux-surround sound in that since they are pointing off to the side I can hear things as if they're coming from that direction.

It's not really worth it for stereo speakers, but if you use headphones a soundcard does have some benefit. I have a cheap Xonar DG soundcard, the headphone amp and virtual surround makes things sound a lot better.