It matters because miners do not have an economic interest in forcing end users off the network. End users provide fees and demand for the currency which is how miners make money.
But if BU will switch to the longest chain eventually anyway, then what exactly is the point of your limit?
Your limit is a function of the resources you want your node to consume. Above that limit, your node either rejects large blocks that are spam (not accepted by the longest chain), or drops off the network because it can't keep up.
You can't blindly accept a 2 Mb block just because it's below your limit, as it might be orphaned because no other miner agreed on it.
How does this differ from any other block?
So you will still have to wait like 6 blocks.
I don't remember seeing anything about BU promising faster (statistical) finality than regular Bitcoin.