455Grandville,
The 6.3s are magnificent - and have one of the most visually beautiful engines I've ever seen (though I admit I haven't seen a great many engines).
Here's a link and a brief quote from that link:
http://en.wikipedia.org/wiki/Mercedes-Benz_450SEL_6.9
Autobahn cruiser versus race track
Top speed was factory-rated at 140 mph (225 km/h), but some journalists testing the car saw speeds approaching 150 mph (241 km/h). Among those journalists was Brock Yates. Yates was approached by the factory to write promotional literature about the 6.9. He agreed, but under the condition that he could list the car's faults as well as its positives. Daimler-Benz agreed in turn, and Yates was given a US-spec 6.9 to drive from Manhattan to the Road Atlanta grand prix race track in Georgia.
I've actually got some of the lit advertising 150mph, but didn't want to go to the trouble of scanning it and posting it.
In any event, the 6.9s were very fast cars considering that most cars had gotten REAL slow by then, and featured a high top speed. As for how fast mine is, I'll never know; I strive to drive the speed limit. My real measure of performance is getting past other cars in traffic to get into the right or left lane when I need to. The other is going up hills and being able to "kick it."
I got my Mercedes in an incredibly lucky stroke. It did not have the 6.9 trunk badge (which was an option for the 6.9, though it could also have fallen off or something), and the seller didn't do his homework. I initially thought it was a 4.5L version, but as I tried to buy parts, several didn't match up. I looked up the VIN, and whadya know? To be honest with you, it was a pretty darn good deal even for a 4.5L model.
The car turned out to be a one-owner medical doctor's car, save for the (most likely a broker) who bought and sold it in a quick turnover. All the service records were in the trunk.
One of the reasons I wanted to get my hands on the Pontiac is so I wouldn't put too many miles on the 6.9. Another issue is the fact that, while the 6.9 has hydropneumatic suspension, it is rather costly to maintain.
Oh, wait a minute. This is a PONTIAC forum, isn't it?
The 455 engine sounds like it would be ridiculously easy to build up compared to the 6.9. It would be VERY expensive to do much upgrading on the 6.9. The other thing about the 455 in the Grand Ville is the fact that you'all tell me it has a roomy compartment; the 6.9L in the 450SEL engine compartment is very tight, with even routine maintenance becoming an adventure.