Would love to see your math on that since from what I've seen and read everywhere Chainmail shits all over porthos. If you want more haste that bad then use something good Ie:
Thaumas coat
... but it looks like Porthos at most is a 10.43%~ increase, while AF1+1 is an 11.63%~ increase from the dual wield.
P.S. Use Thaumas.
Comparison using my spreadsheet:
With 52% total DW and 21% gear haste, plus the Haste spell, using Blade: Shun and whatever other gear was in place, I get:
Iga+2: 227.231
Ninja Chainmail: 236.335
Porthos Byrnie: 236.629
Thaumas: 251.712
So, Porthos and Chainmail are almost identical (exact variance will depend on which weapons are used, though they tend to be within about 1 point of each other), and Thaumas wins.
Add Marches:
Ninja Chainmail: 313.223
Porthos Byrnie: 325.435
Iga +2: 332.517
Thaumas: 346.910
Porthos beats Chainmail (not surprising; hitting delay cap), but Thaumas and Iga+2 beat both.
So Porthos is pretty much just a sidegrade to Chainmail; not awful, but not an improvement either. Euxine +3 and Thaumas are both improvements.