AFAIK there is only one mod that yeilds any noticable return on its investment.
I have herd (But not tried, so I cannot comment on them) that nothing really helps much on the 2.5 =(
The Inertia Ring seems to be the only thing that gets 100% positive returns on the investment.
I have a 2.5L, rod is a knockin and will rebuild it unless a 4.0 comes across for free.
I can't get over your statement, while you say you haven't tried anything else you also say that the only mod that gives any noticeable return is the inertia ring and you mention 100% return which i don't really know what that is for sure it's not horsepower. So to make it clear, adding mass to your flywheel will make you lose horsepower and for street and hwy driving will not improve anything. if you look at any performance crankshafts and flywheels they are lighter and thinner than stock, i'm not saying the inertia ring is a bad thing but for sure is not a performance mod.
to have more available power you need to either eliminate some losses (have less power go to waste, that will also improve your mpg) or make more power out of the motor (and for that you need more air and fuel - you cannot make more power without adding those). There are also drivetrain improvements but most benefits are given by proper selection of axle R&P ratio.
It should also help on hills or anywhere else, you need momentum. It takes torque to rotate the extra mass not HP.
power is a function of torque multiplied by rpm.
EDIT: Power_HP=(Torque_ftlb*RPM)/5252