All of this depends on engine condition and you can't tell by a simple compression test. It will give you some idea but I tend to go by several factors before I recomend any high or prolonged RPM useage. If it's an old original engine and it looks beat up, I say NO WAY!
I've had several engines fail by higher RPM shifting or prolonged RPM over 4,000 .
In one case a rod bearing let go. I could hear a faint rattle that got louder as I drove along after taking it up over 4.500 just one time while shifting. This was on a 220SEb coupe which is very similar to a 230SL engine. This engine ran well enough but actually had a lot of wear on all componets.
The other engine failure was on a 250SE coupe which is the same engine as a 250SL. In this case I was on I 75 coming back from the Mountains. I managed to get home with the car but it failed the next day after I got home. This engine also ran well but I never drove it hard and kept it around 3500 or less for the year I drove it. On the interstate, I had to keep it at 4,000 or more to keep up with traffic. Thinking I was OK I did that for hours at a time. The engine diied from terminal '' death rattle '' and I still have the piston that I use as an example of what can happen. This engine is now in the Red Rocket after being rebuilt. In this car I never have to drive it more than 3,500 RPM to keep up with traffic because it has a 5 speed. That alone is worth a lot in terms of engine wear and longevity.
Before I changed the engine in my car it also died from death rattle and it never saw high RPM.
So, how fast it is too fast? You won't ever know that until it's too late if you believe any speed is safe. Yes, you can spin these engines very fast but only if they're in solid condition. Any cylinder or wear around the piston rings and you will eventually break a top piston ring at high RPM. Keep it under 3,000 and you can drive even a worn engine for a very long time.
The one thing I'd bet my reputation on is you CAN'T run any of these engines at high RPM's for extended periods IF they're worn out - they WILL spit out a piston or rod bearing.