Purchased a bull 24in shilen 17 rem barrel.  I know that 7056 is stronger 6061.  Would I benefit having a billet CMT slick side upper over a standard milspec?  I've heard conflicting stories.  I can see the argument for long heavy barrels in the ar15.

Sure, stronger is better on paper.  In the real world, you probably won't tell a difference.  I can't say that I've heard about 6061 upper receivers falling apart on guys cause they were so weak and floppy.

I do think that some upper receivers are better than others depending on their job.  For my rifles, I feel like Aero M4E1 and M5E1 are better because they mount the handguard to the receiver instead of the barrel/nut.  For my Glock mag ARs, I like the NFA/Joebob uppers because they have LRBHO built into them.

I think you'll see more differences between the slick side vs milspec part of your question before what grade of aluminum they're made from.

I agree.  I was referring to the rigidity of the upper to help support long heavy barrels as an accuracy point

Robert Whitley believes larger/heavier uppers are more accurate with longer/heavier barrels, so I believe it.

If you’re talking about JUST the upper receiver itself, it’s a different question from if you’re talking about complete, barreled uppers.

If you stick with a well known brand, it’s hard to beat a forged (stripped) upper.  They are mechanically stronger, not just because of the stronger alloy used in most forgings, but because of the forging process itself.

Forging into the rough shape of the upper builds in strength in the critical areas such as the front, threaded part of the upper.  If you’re planning to build an upper with a long, heavy profile barrel, I think you will see a difference in the stiffness of a forged upper compared to the typical billet upper.

To me, billet uppers and lowers can provide a few features that aren’t usually available in forged receivers.  My Glock-specific lower from Quarter Circle 10 is billet because of the changes they needed to make in the magazine well area to accommodate Glock magazines.

On the other hand, I see a lot of billet parts, mostly lowers, that seem to be an opportunity to “express the artistic side” of the maker. To me (PERSONAL OPINION), skulls, flames and shark teeth belong on motorcycle tanks, not ARs.

If, on the other hand, you’re talking about complete, barreled uppers, YES there are differences between makers, their level of experience and attention to detail, and the parts they choose.  I have bought only a few complete uppers, and only after thoroughly researching the manufacturer of each, because I have built enough uppers to know how easy it is to goof - or to cheap out.