My understanding on ports are that the top 3 ports are the ones responsible for the timing charachteristics of the engine in conjunction with exhaust and inlet timing .
The lower ports are traditionally used as relief ports. When on the down stroke the fuel can flow out of the side of the sleeve as opposed to the bottom of the crank case , this can help with top end but is not manditory to achieve it
If you use efra approved 3 port .12 engines as an example they are all 3 port engines yet have different top end and bottom end ... All their differences are created by port timing changes. And .12 engines normally rev out higher than .21 's etc .. I do realise this is because its a smaller engine cc but they have no problem getting up there in terms of RPM ..
MM