Yes, you should definitely avoid using the sp_ prefix. Not just the performance hit, since it always checks the master and resource databases first. Also, the next version of SQL Server could have a new system stored procedure with the same name; you'ld be scratching your head for a long time to figure out why you application was suddenly failing.
However, as a DBA, you can be comfortable using the system stored procedure, even in a develpment database. A programmer can't trip you up by leaving an sp_ stored procedures waiting for you to execute it with SA rights.
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