The 5.0 litre revs much better than the 5.6 litre but in NAM export detuning the 5.0 litre lost so much power that the 5.6 stroke increase was first produced specifically to substitute for the 5.0 to maintain luxurious power-weight performance for the American market. The American 5.6 moves a heavy SEL and SEC luxury chassis roughly the same pace as a Euro 5.0.

So where the story gets interesting is in the SL versions designed for more performance on the chassis side. Mercedes never offered a 5.6 version of this in Europe specifically because in the lighter chassis the 5.0/245hp Euro motor revs better and moves the car just as well as the 5.6/280hp despite a power loss, due to greater engine flexibility with oversquare dimensions. Regardless of having more power in Euro trim the 5.6 next to the 5.0 is loosely speaking like comparing a tow motor to a performance motor once you start opening them up to produce some power. The 5.6 has piston speed issues, throw weight, crank flex and you don't get any of it in the 5.0.

That said, in NAM detuned versions there's no benefit sticking with a 5.0 over a 5.6, hence in the SL series the NAM export came in a 560SL which simply doesn't exist in Europe, it was only ever produced in the detuned NAM export version and that was purely to offset a dramatic power loss in NAM detuning without having to build a whole new motor just for Americans. German car manufacturers are, after all limited production capability houses.

But that's if you're going to keep a stock or near stock M117. The upgrade over a stock NAM 5.0 would be either a NAM 5.6 or get a pair of Euro cams for your 5.0, it's the only main difference between Euro and NAM versions, given you're going to replace the exhaust anyway and that was different too, plus I'm not sure if NAM 5.0 has twin intake tubes to the air cleaner or one blanked out but all Euro M117 have two.

If you're going to do more performance work to the engine, such as custom performance cam grinds and piggy back the ignition, do some machine work on internal parts or anything like that then definitely keep the 5.0 capacity. Both the 5.0 and 5.6 will return around 300hp with a mild working over but the 5.6 will act more like a tow motor good for a big SEL with a wine fridge in the back and the 5.0 will spin far more freely which is better for an SL or a stripped street car.