I installed new coax speakers in my doors.
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Should I invest in tweeters to replace the stock ones ? Or mount some in the A pillars ?
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or ?

 

Your call, but me personally, I would ditch the coaxials and go with components for the doors. The woofer mount locations are down low, making acoustics for the "highs" from the tweeters less audible. For that reason, when I updated my stereo, I used components and kept the factory tweeter locations, I'm glad I did. The sound stage (acoustics) with proper components up front gives a much better audio experience.

For context, I'm using Focal ASE 165 S components. Nothing crazy powerful, but they sound fantastic running straight off the double din Kenwood I'm using. The S is for "Slim" these speakers have a much shorter depth allowing for easy clearance in the doors to avoid contact with the window glass. I made a detailed post on my stereo install with photos.
Cheers!
-Icer

 

I agree with Icer and think the tweeters are a critical part of the sound system. The MB Quart separates in my front doors opened up a world of sound after getting some clean power from a two-channel USX-2100 Amp. It's old school, but the tweeters are well placed in the doors and provide superb highs. While I eventually used MB Quart coaxes for the rear speakers (and rear fill), and the system has held up pretty well for over two decades.