They are tweeters that would be there. The base system isn't actually that bad. Like you I assumed I'd need to upgrade it but I've not bothered as it's just about ok.
A place near me specialises in BMW upgrades and you'd end up with something much better than HK. The only downside other than cost is them fitting the amplifier in the boot.
https://www.audiofile-incar.co.uk/au...7-and-x-series