Question: Baked desserts from home and fine restaurants leave no bad taste in my mouth. But catered deserts at my workplace or those purchased from a grocery store leave a nasty aftertaste that I can't stand. I've noticed this consistently for years with food supplied by multiple locations. Frequently I resist the temptation to indulge because the yuck factor after is not worth the initial yum. What ingredient could it be that leaves such a terrible taste in my mouth that has me scrambling for a toothbrush and toothpaste?

Answer: 
If you think of tenderness on a scale, sirloin and tenderloin are nearly on opposite ends of the scale. Sirloin, in general is not a tender cut of meat. Your result has little to do with seasoning or location in the water bath...or even cooking method. Sirloin is a lean and tough cut. Often sous vide can be used to make tough cuts of meat more tender, but sirloin doesn't really have the collagen or connective tissue that will break down like, say, a short rib. If you like the tenderness of a tenderloin, you will never match it with a sirloin. Don't blame your circulator, it is akin to blaming your stove when things don't go well there.
I had two identical experiences in the last month with grass-fed and followed up with a trusted butcher (he did not supply the chewy meat). 

Aging of the sirloin was to blame in my case. There was no hint of how long the beef was aged in either case, only sell-by dates. I did have a chance to ask one of them and it turned out to be about two weeks by the time it reaches the counter, and the ones from the trusted butcher were at least twice that. I am not sure if that is the only reason.

15min extra sous-vide time should not make much of a difference for a sirloin.