i have a 99' 220 and did a rebuild and mods a short time ago. this involved replacing the old stock piston with a wiseco unit, changing to a 35mm (ex 200) carb, boyesen power reeds, and some porting work..
all went very well but i should mention that the porting was more clean up the ports type work done by myself not a shop. also i didnt change the exhaust,sub exhaust or transfers rather i just spent time cleaning up the casting marks in the boyesen ports (the 2 ports next to the main intake that lead into the transfers) and i reshaped the intake port bridges to provide a better intake flow. i have done this type of work before on other bikes so had a basic idea of what to to, plus on the team green website it shows simaler work to be done on the kx bikes.
all up this combination of mods has made my little 220 quite the screamer... it now still has good bottom and mid but also a healthy top end which you can really wind out, in fact the top is so good i now really need to lower the gearing as the gaps in the gears can be a pain..
anyway to cut a long story short im happy with my bike, but as to weather you go ahead with porting i suggest its better done in combination with other mods and also you need to be sure if the powerband changes are what you are really after i.e. would it be cheaper to sell kdx and buy a kx etc..
good luck.