You don't have to go multiple instances for load identification reasons.
As Andy pointed out, there are several reasons not to.
You just setup SQL profiler trace for slow running queries (that will show blockings also), save it to a table and run it 24/7 . The trace log has the db info as well, so you can identify which website has the load activity going.
Once you have the trace in a table, you can periodically examine it for slow running sprocs & queries. Generally you keep the logs for a month, but you can keep it for a year if you prefer.
Kalman Toth - Database, Data Warehouse & Business Intelligence Architect
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