There's some misinformation here I think from overthinking and generalizing. Porting is decidedly not one-size-fits-all.
The way porting levels work is this:
Stage I - The biggest bangs for the bucks come early in the process where the seat angles and approaches are optimized and casting flash and bumps are removed. That helps low lift flow fairly dramatically. It's almost all able to be done by having the basic work done to a very high level of quality (the 5-angle valve job, on top of the line equipment), followed by some work with a die grinder. The initial cleanup work does not negatively affect anything, in fact it picks up reduces the response time in a port reducing mixture seperation issues. It helps with any camshaft size. It's as easy as choosing the right shop.
Stage II - Includes blending the bowl, tweaking the port features, and gasket matching the intake side. That does add a small amount of volume to the port, but it's about streamlining and removing turbulence - not "hogging" out. It's pretty hard to mess this part up unless you slip with the grinder. It adds more low lift flow and begins to really add flow at mid-lift but normally does not add much to the higher lift ranges. It helps with any camshaft size. It's not much more $$ than the first step, could be done by the end user - that's about what the MP porting templates were for.
Stage III - Includes reshaping the port a little. That includes changing the shapes in the short turn and in front of the bowl, changing the guide boss, the pushrod pinch, and the roof among other things. This does increase the volume a bit, but one of the differences between poor, good, and really good porting is that increase in volume. Depending on the intended use, the engine, the castings being worked on, etc - the volume and shape's what can make or break the job and it's this level that normally does affect the low speed performance of the whole package as a compromise to increasing mid and higher lift flow windows. This level needs to be matched carefully to the engine, power levels needed, and the camshaft. It takes a bit more money than the first two, and yields a little less bang for the buck but this is what you find in the CNC programs.
Last is all holds barred race porting which can include modifying the head and going for max flow at high lift. It costs whatever it cost.
Each level is dependent on and includes the previous - so a Stage III will include a really good valve job and basic cleanup. Stage one is the cost of the valve job. Stage II adds usually $250-300. Stage III will add another $600-700 to Stage II. Race porting can go well into the thousands. Ask a winning FAST racer what their heads cost...
So in my opinion you don't need new heads, especially ones that aren't much better to start with - you'll have to spend to get them really good, and you can just do that with what you have.
