Personally, I think the soundbar is better than upgrading the dash, if you could do only 1.
The soundbar speakers are much, much, much easier for me to hear due to their location (and possibly size).
Like on a scale of 1-10 for volume, I'd say my fronts are a 3-4, while my soundbar is a 7-8.
However, it may be worth noting that my audiophile friend gave me some lecture about proper soundstage requires the speakers to be in the front.
My "soundstage" has me hearing the most sound from the rear. He hates that. I'm fine with it.
(Yes, I can change the fade...)
A sub will also make a significant improvement.
Note: I have 5" in dash with adapters in front and I think 6" in the soundbar. I might be off the number a bit, but the soundbar speakers are at least 1" bigger.
Also do the foam/baffle thingie.