Base sound system upgrade questions

I have a 228i with the base sound system.

I'm thinking of upgrading the speakers and possibly adding the A-pillar tweeters as well. I searched and it looks like most people are upgrading from the HK or hifi system - but I'm at the base.

I saw that ECS tuning has the pillar speaker parts.

Can anyone give me some info on where to get started? Are there any upgrades from base that you've done that you're happy with?

Base sound system upgrade questions