how much power will stock rods survive?

340srule said

I'm curious are you saying they're no good from personal experience or going by what you hear??......

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Both.
I read about them after the shop had used 2 sets. What we did notice was there was a .003-.005" variance in center to center length between 10 of the 16, and 9 had issues with being out of round on the big ends after being cycled. The pin end always needs honing, so I dont consider that a problem. They were installed, and as of this point (2 years later IIRC) they have served well. After reading of other failures and agreeing with the failure analysis of people much better than I, I decided not to bother with them. I use basic specs as a good indicator of quality control in all areas of manufacture. In this case for me, they arent "good enough".
The cost if I were to buy the SIRs today is $240 to us. Add $100 for the labor to fix them. We can get a set of Eagle H beams for $430. The H beams I've done (3 sets) have all been within .002" (in terms of center to center length) of each other, and after cycling have no issues with the big ends. They are also stronger than an SIR. Interestingly, the new Mancini site says "use under 500hp and 7K rpm" for the SIRs. My mild 408/416 combos are making over 450. So even if the SIRs are good after cycling, I'll pay the $190 more for the stronger piece. It's cheap insurance for me when spending someone else's money.
You are not alone in having these rods work for you. It's no reason to get defensive. I just choose to use what I consider a better rod. In milder std stroke engines, I use factory rods with good bolts all the time. The 340 rod is a stout part to begin with, and I've had heavy piston 340s I personally bounced off the rev limiter at 7200 once or twice...(not on purpose, but they did it...lol)