Date registered: Feb 2008
Vehicle: 2004 W220 (S430) ; 2001 R129 (SL500) Sport
Location: Alabama Gulf Coast--formerly Charlottesville, VA
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I'm sure you realize the swap you are considering is WAY more complex than taking out one engine and dropping in the other. I don't think the transmission in your 2.8l is compatible with the bigger motor, so that will have to be changed, which implies the driveshaft will have to be altered, which means the differential may have to be strengthened. All that assumes the engine compartment will not have to be physically modified, motor mounts not will have to be relocated, the transmission tunnel will not need modification, the electrical harness will not have to be altered, that the exhaust system would not have to be modified. The list is endless.
If the swap was a drop-in, then you would have a very fast C-class without the suspension or brakes to cope with the power of the engine.
The answer to your question is YES, it would be significantly faster. So would a Cooper with a bi-turbo V-12, but you don't want to try that swap either. You would be much better off and spend a lot less money to sell your C280 and buy a C43. The $3500 for the junked motor is just the tip of the iceberg, assuming it doesn't need significant repair.
"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck