Me personally I would want to set the tune myself as I do not trust how it is set from the factory. My 620 that I just bought was pretty lean on the L side of things. It would struggle to idle at times especially when it was cold and was really boggy when hitting the throttle from a stand still. Clipping the tabs off of the adjusters and re tuning it really made a difference in how it starts, runs and idles. It is pretty hard to get these too lean on the H side from what I understand as they have the bypass jet on the high side, but I was able to set mine yesterday how I always do, which is set it so rich that it 4 strokes in the wood and then dial it back a little at a time to where it just cleans up as soon as it is in the wood.
Any time you increase exhaust or intake flow you are going to be changing the tune of the saw, for max gain of your mods and the longevity of the saw I would recommend retuning when you do the mod. I am going to put the red beard air filter on mine as it is already trying to suck fines in by the intake tube after 2 tanks of fuel, so after I do that and get about 8-10 tanks through it I am going to pop the muffler off and open it up a bit.