Hi Martin. For what ever its worth I rather doubt that MIR will cause a tsunami. While it may weigh 40,000 tons it would start breaking up long before it impacted the ocean.

If an asteroid of size 200 meters hit the ocean the tsunami it would create a tsunami. If an asteroid of size 1-kilometer hit Earth, it would cause a dust cloud, which would block out sunlight for at least a year and lead to a deep worldwide winter, exhausting food supplies.

The thing to keep in mind that while MIR weighs 40,000 tons in space by the time it impacts the ocean most of that weight would be bits and pieces. In affect there would not be a 40,000 ton mass hitting the ocean as a total intact mass. Just my humble opinion. Take Care…Don in creepy town.