The main things to look into improving in the 500/550 head are pretty straightforward. Since these engines let you pull the head in the frame, it is easy to do-a-little, ride-a-little, repeat until it suits your taste.
When you look at the ceiling above the intake valve, you will see it forms a tight wedge into the valve seat. This particular area is responsible for killing the once-excellent air inertia from the long intake runners back to the carbs. If you instead pocket it a little right here, the air mass will compress itself somewhat during the snapped-shut valve time, and will be more eager to enter the cylinder at the next intake stroke, still being at higher pressure when the valve starts to open. The total volume should not exceed the volume of the port for the same depth upstream, though, or the flow will stall and a flat spot will develop in that RPM range. This is known as "over-porting".
To then take full advantage of this compressed intake flow, the edges of the valve seats insdie the combustion chamber must also be cleared (and sometimes the nearby cylinder wall moved back a nick or two) or else it takes longer to start moving. A small amount of change makes a real noticeable performance change, simply because these engines were made in molds where this was too difficult to [manually] control in the 1970s for their cost targets. By the time of the 750F1 the molds and ports had become works of art, but that never quite made it to the Mid four bikes (it did to the CB650 and 400F, though!).
