I'm not too surprised to be honest...golf line distance with a similar understable type disc likely won't change all that much. If you want to throw further without upping disc speed or throwing faster (ie. increasing line drive golf distance with all discs), likely the option is to throw high hyzer flip/turnover shots that really glide left to right, or experiment with higher flex shots on stable discs.
I'm in the same 375-400' usual range as you, and I just find that the discs don't make much difference distance wise (line/consistency is different). One day I'll be throwing all my drivers 375, another I'll be throwing them all 400+. Stable, a bit overstable, understable, they all move +/-30' or so together depending on my power that day. As long as you aren't throwing something that's flipping into the ground or so OS it's fading out 40' before your other discs in your power range, I'd say that getting another 30-50' is going to be technique and not magical plastic choice unless you try to get used to a Tern class disc and see if it helps you at all.