It's good that you know how to service them etc. But what worries me is that amount of pulls that mine take to start, as well as the power loss that they have. Just out of curiosity, are these types of problems fixable if you know what you're doing?
First, I see that you had a Shin 270 stolen, man that's bad! The 270's are rock solid--and for the most part repairable. Mine came with a stripped shaft, clutch and clutch drum were gone, but it ran--and I traded 1 hr repair labor for it. $100 in parts and I have a great trimmer.
Before you do any of the steps below, try some "LMT" first. You can get this at Lowes, and Tractor Supply. It's an aerosol, yellow top can, found in the lawn equip section--usually close to the sparkplugs. Follow directions on can. Spray into sparkplug hole, and gas tank--again just follow directions. I refer to this stuff as "miracle juice".
For loss of power--make sure muffler and exhaust port are clear--this is an easy check/fix. Pull muffler, burn with a propane torch if you have one--the idea is to burn out the carbon. If exhaust port is clogged with carbon, move piston to bottom of stroke and clean port with something wooden (my grandkids save popcicle sticks for me)--don't use a screwdriver, you can scratch the cylinder or piston.
Check/change fuel filter and fuel lines if theres any doubt, and maybe even the plug just to make sure.
Carbs aren't all that difficult--just several tiny parts--don't lose them. Take apart, clean with carb or brake cleaner and blow out with low pressure compressed air. If you have one that runs better than others, switch carbs and see if it changes how they run. Repair kits for the 270's are fairly expensive--don't know about the ones you have. My success rate with carbs is certainly less than 100%.
Hope all this helps!