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I don't think the weight itself matters much for reliability... not nearly as much as the OAL and bullet shape/profile. In fact, a change in lot number alone can be the difference in a few thousandths OAL (long or short) or crimp, and that can make a difference.

Regardless of how many rounds you shoot before you trust the gun, make sure you shoot a qty or your SD ammo and any subsequent SD ammo you want to carry. Because almost NO 2 JHP's are alike. And even 2 that look very close, one may not even feed while the other runs like a sewing machine.

Unfortunately, the site does not have the exact same grain (124 grain HST +P) that I previously purchased and started qualifying with. They do however have the larger 147 grain HST +P) in stock.

Am I being too crazy about this? Does it really matter if they are from the same manufacturer, same load (I assume), just different bullet weights? If I fire the 147 grain HST +P, should I call it good and carry on?

The 147gr projectiles are longer than the 124's... and ammo is obviously of different lots. So the OAL may or may not be the same, crimp may or may not be the same, etc. Also, they are not the same load... the weight difference requires a different charge- they could use a different charge weight or a totally different powder altogether.