What year is your car? I think the only flaw in the M103 is that pre '89 engines are subject to worn valve guides. The more high speed use the quicker you will require a valve job. Mine is a '90 and needed the valve stem seals replaced. Easy job, I did it myself in a couple of hours. That cured my oil consumption issue. Otherwise the M103 is bulletproof, definately one of MBs best. My Gwagen wieghs in at over 5200 lbs and is geared rather short. I have taken it on a few long distance highway trips and it has been flawless. This summer we drove from MN to CO across Nebraska. It was 105F and very humid, the G ran 80+ mph with the AC blasting for 6 hrs. At those speeds the M103 was turning in the area of 4000-4500 rpm. I have also driven it across the So. Cal. desert at similar speeds with a rooftop cargo box and had no trouble. It has always run cool and smooth. I suspect the Gwagen is about the most strenuous application of this engine and I have been very impressed with it. In a 190 I would not hesitate to run it flat out, it is a very tough engine. Over the years I unlike you have always run my cars at thier limits. I remember my mothers '85 Accord, doing backwards donuts in the rain and accidentally seeing the tach swing to 8000 rpm. The car had 10k on it at the time and I sold it in great running condition with 165k on the clock and no engine work. I also run my Porsche very hard, it sees 120mph plus every day I drive it, it also visits the track 3-4 times a year. It goes through brake pads and tires but the engine is very strong still with over 70k fast miles. IMHO you have nothing to fear, your car will run as fast as you dare for a long long time if properly cared for. Take it out and run it, thats why I buy german cars, they seem to revel in it.
90 300GE 5sp
86 944 Turbo