I've been a Stihl tech for 34 years. I came into the industry when 031's and 056's were the hot saws. Been there, done that. Mtronic is an excellent engine management system within the limits of its capabilities. What nobody here has addressed is the original issue "the engine runs too fast at idle". Let's put Mtronic aside for a few moments and think about how a competent tech would approach this problem on a non Mtronic saw.
Stihl has a wonderful diagnostic flow chart called "Stihl engine checklist" or something like that. Its simple, but long. Most techs don't bother with it until all their guessing leads them nowhere in solving a problem. I use it when the answer isn't quick and obvious, and it has never let me down. Again. we're pretending this is a standard carbureted saw with a high idle. Most likely, we'd all try adjusting the idle speed screw to correct the problem. I would. But what if that didn't work? Then it gets interesting. We need to systematically look at all aspects of the saw to determine the problem. The diagnostic chart will take us through the fuel system, controls, ignition, exhaust, etc. Lets say that all these items turn out to be ok. The test that will probably be most telling is a pressure and vacuum test of the crankcase. Even a small air leak will cause a saw to run erratically or not at all. High idle could easily be explained by a leak at a gasket, seal, impulse line, or intake boot. If there's a leak, fix it and re-test the crankcase. Now yo can properly set the mixtures and RPM so the saw will run properly.
Now lets go back to an Mtronic saw. Everything thats critical for a carbureted saw to run properly ALSO APPLIES to Mtronic. If you have an obstructed fuel filter or a crankcase air leak, that Mtronic saw won't work properly. Its critical to be sure there are no mechanical problems prior to blaming Mtronic.
Has anyone checked the mechanical parts of this saw, or did they just skip all that because "they knew" its an Mtronic problem???? Mtronic isn't perfect, but it is a simple, generally solid system. What it can't do is magically retune a saw that has an air leak or other mechanical problem. Once all mechanical possibilities are ruled out, only then does a competent tech look at the engine management system.
Hook it up to the laptop and run SDS software. Look at the engine management data- number of starts, total hours of operation, and most importantly- the fuel numbers at idle and WOT. If Mtronic is screwed up, the data and solenoid tests will tell you. If its compensating for an air leak or other mechanical problem, the mixture data will be crazy rich. Mtronic is remarkably simple and usually works perfectly. Sure, there are issues with very early controllers whose firmware was set so that the saw would run too lean, but Stihl has addressed those issues with newer controllers.
Mtronic is electronically simple, but dependant on lots of mechanical things to work properly. A leaking inlet needle in the throttle body will cause the same problems on an Mtronic saw as on a standard carbureted saw. Crappy fuel still doesn't burn well in an Mtronic saw. Water doesn't burn at all. Look at all the simple, normal reasons for a particular issue before jumping to the possibly erroneous conclusion that Mtronic is to blame. My time is valuable, and so too is yours- why waste it guessing. Approach a problem with a systematic plan and rule out problems rather that rule them in. Think about that for a few seconds. Those few seconds of thought may save you hours of frustration,
think i'll stick to the 056 that has me confused with all the different changes in parts etc. 
