What might make Muscle scale better into the late game would be to change the +1 Attack_Pierce to say...
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_StrengthMod</StrVal>
<Value>0.1</Value>
<Provides>+10% to Strength Bonus to Attack</Provides>
</GameModifier>
Assuming this stat still does stuff, of course!
<UnitStatType InternalName="UnitStat_StrengthMod">
<DisplayName>% Strength Bonus</DisplayName>
<DisplayNameShort>SMOD</DisplayNameShort>
<Description>Multiplies the units strength bonus to attack.</Description>
<Hidden>1</Hidden>
<DefaultValue>0.0</DefaultValue>
<UnitStatGrouping>CombatStat</UnitStatGrouping>
</UnitStatType>
I was actually looking for a catch-all Attack_Pierce multiplier. That's the closest I see in UnitStats at the moment.