Don't use optional rules you think are broken?
While I agree that the piecemeal armor rules need to be adjusted to prevent piecemeal armors from being far better than normal armors*, I'm not sure that adding complexity to the system is the right way to go about it.
* OP's armor is a 9 AC heavy armor with a +3 max Dex that costs 250 gp
OP = Original Poster
Piecemeal Armour is very realistic.
Heck, I have a armour set that grants the same bonuses for 225GP. It is based on real armour worn during the Pike & Shotte Era.
Plate Mail was expensive and as such was a Status Symbol more than Optimal Cost:Protection Ratio.
If you don't like it don't use it. But make them have to pay for any Adamantine/Mithril/etc as a Whole armour suit NOT the individual Pieces. Use the Heaviest category of armour.
So your example would be Heavy Armour.
Also once enchanted the pieces lose all magic if they aren't as a whole can nerf it as well. Though I tend to base the Magic on the Torso Piece.
And Remember: "What is good enough for the Goose is good enough for the Gander." If PCs get this rule so do NPCs. It actually allows Low Level NPCs stay competitive longer. & it is one of the few systems that give a Boon to Martial Characters and adds a little more variety.
I come up to a +8 armor with max dex +3 and -4 check pen.
I think its mainly chain and plate torso pieces that skew the system, because they are supposed to be the same as the regular torso-only armor pieces you can buy normally.
Chain & Plate are potent and with good reason.
Cost: 130, +7 armor, +4 max dex, -2 check pen, Spell fail 35%, speed 30ft., weight 35lb. Medium Armor. non-masterwork.
I have found my low level boss' new armor!