Let's say you have 1,000 parts all based around a base part, which is moved around quite frequently. Would the 1,000 parts have their new properties (CFrame, Position, etc.) sent to the other clients, or would ROBLOX simply send over the new location of the base part and leave the rest of the client?
I recognize that this is a probably an extremely hard question to answer, but if anybody could answer it for me that would be amazing. Thanks guys!
You are basically correct, parts that are welded together (or may satisfy Roblox's criteria to treat multiple parts as one another way) are updated as if they were a single part.
Source: http://blog.roblox.com/2012/12/featherweight-parts-one-million-parts-one-roblox-world/ "Our solution allows us to group these parts together into batches..."