Well, like I said before, a 6 HP carb is most likely going to be the same as a 5 HP carb. The OHH50 - OHH65 (rated 5 - 6.5 HP) all have the same carb, and I think an H50 and H60 carb is the same. If you have a non adjustable carb, a adjustable carb will be a big upgrade, and will be better suited for tuning the engine for more power with the straight pipe.
Find out what it is for sure, and we'll have a better idea of what would be an upgrade. Below is are pics to find out whether you have an adjustable or non adjustable carb.
Adjustable:
Non adjustable:
The easiest way to tell is to see it has the adjustment screw on the bottom. A non adjustable carb will appear to just have the brass bolt holding the bowl on.