Having gone well over 130 in my 4.5, I find it hard to believe a newer MB chassis with an "Improved" V8 (debatable) could not SURPASS what I've done in my 4.5 - so 140+ should be as easily attainable for a 126 V8 as mine, right?

Definetly sounds like either you're way too rich, or your timing is way too late. If they had the front cover off, they could have set the distributor a tooth off and not been able to get the proper timing on it, and just left it. I dont know if late cam timings would produce that high RPM stutter on a 5.0 m117, I know on my 4.5, with late cam timings, it was sluggish at low RPMs but still had plenty of power in the higher ones. They might be too early if anything. But check your timing!