How do I best measure the query performance?

I like to use the SQLQueryStress free tool when comparing a before and after scenario. With SQLQueryStress you can execute each stored procedure as many times as you like, and get the total average stats for all executions.

For example, you could execute each stored procedure 100 times, and then use the stats to back up your improvements. "Over 100 executions, my improvements save a total of 30 seconds and the stored proc does 1500 less reads per execution." I think you get the idea.

If there are parameters in the stored proc, it's always a good idea to double check that your improvements work with many different sets of parameters. SQLQueryStress does some cool stuff with letting you substitute parameters in your query to get a better overall picture of how the stored proc might be performing.

SQLQueryStress documentation: http://www.datamanipulation.net/sqlquerystress/documentation/documentation.asp

4/ You can go to http://statisticsioparser.com/statisticsioparser/ and paste your stats in order to see the overall score.

When you have collected execution times over a couple of days for your two stored procedures, I will recommend that you use this homepage

http://www.evanmiller.org/ab-testing/t-test.html

to see if they are actually different.

6% difference does not sound as much, when it comes to improvements of stored procedures. I've come to expect two orders of magnitude from my colleague, and I pretend to be disappointed if he only achieves one order of magnitude...

He does not have to use the EvanMiller homepage to prove that his solution works faster.

I would also install SQLSentrys (edit:) Plan Explorer from http://www.sqlsentry.com/ as this is a much improved tool for comparing execution plans.