Absolutely a Simulacrum can gain in power, using the Rules

[smuckenfart]

If a simulacrum is no longer a simulacrum, it is not bound by the limitations of a simulacrum!
Polymorph any object changes a target creature or object into another.
A polymorphed simulacrum is no longer a simulacrum, it becomes another creature or object.

A troll polymorphed into a human loses its regeneration.
A simulacrum polymorphed into a human loses its inability to gain xp.

Now you have a cohort duplicate of yourself so long as it isn't dispelled.

 

[aco175]

Sounds to me like you are trying to skirt the rules. You are trying to use the general polymorph rules to get around the specific simulacrum rules.

 

[smuckenfart]

Using the rules to get around it, yes.