Those heads are a low performance head made more for low speed RPM and torque from a Gen 5, if I remember correctly. I've never had a Gen 5 but from what I understand, there aren't too many parts that can swap between the Mk4 and Gen 6.
So after sitting in my boat pretending I was at the lake after going through my flooding carb issue, I pulled off a valve cover to see what kind of heads I am working with...
10114156...91-up...oval...OPEN...Gen.V & VI 454, 118cc chamber
So, are these any good or are they severely limiting any potential of gaining horsepower in the future?
Those are "peanut" port heads, not for performance. Before you consider swapping heads, check the block casting number as the Gen 5 blocks are not good for putting MK IV or aftermarket heads on. Gen 6 works with better variety of heads.
Don't plan on just adding a different set of heads and gaining a bunch of power, the cam needs to match your application to make it all work