1. I run semi for a couple of years now. For me speed is not important, and i did not notice any difference, besides you gain time on cutting, you waste it on sharpening.
2. The length of teeth has no difference in cutting ability IMO. When i sharpen (by file) i run the file as needed for a particular tooth. if one got hit by a rock and needs 7-8 passes, it gets it, if the next one needs only 2-3, it will never get the fourth. If a chain is OK and one tooth is badly damaged or on the end of its life, i just remove it. I just check the rake height. Never noticed any difference. Unless you are a logger who cuts wood for a living, you will not either. Saw also does not have a caliper to check how long the tooth is.