If you have a bog standard SQL2000 function, for the sake of argument it multiplies one argument by another and you write a query that applies that function to records in a table. You get a big performance hit because the function is applied row by row.

If you have your .NET assembly function does the same thing apply?

In general, yes, the same performance hit would apply to CLR functions. The scalar function is called once for each row in the same way as a standard t-sql function.

Also, does the CLR mean that extended stored procedures are now consigned to the history books?

Extended stored procedures are still available. I do not think that many new ones will be created though. There are two factors to consider however. First, for backwards compatibility both the existing xprocs as well as the possibility to add xprocs to SQL Server is needed. Second, as I mentioned CLR Integration is not enabled by default. Therefore there is no CLR replacement for xp_cmdshell, for instance (note though that xp_cmdshell is also disabled by default).