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I have to jump in. I never used a girdle. I never used aluminum caps.
I used old time circletrack cap straps. Early in the life of this build, I was learning the ins and outs of engine tuning. Nothing gives an indication of your tune like the caps. As previously alluded to, similar metals when moving against each other will exchange material and show positive evidence of movement. If you are seeing this, expect failure. You are exceeding the clamping limits of the structure. The peppering on the mating surfaces tells you that the timing is happening to early for the engine. That being said...If you are not seeing it with a factory block, you aren't making all the power you "unreliably" could.
With a naturally aspirated engine, you are limited in the WAYS you can make power. I understand the often thrown around hp limits of factory blocks. I think the generally accepted 750 hp number for a stock block is a bit on the low side, but I'll roll with it.
Getting into turbo apps, throw that number right out the window. A properly(in my head) tuned turbo engine will reliably exceed 1000hp. I was seeing cap walk early in the life of my 470. I was sticking to closely to commonly accepted timing curves. After listening to people with a lot more experience than me, and piecing clues/evidence/info, I started doing my own thing. I would say, any real experienced dyno tuner would easily get more power with less boost out of my engine, but never with the reliability I got out of it. I've had guys nudge me to put timing in the thing, (a couple of degrees could be 60hp) The engine made low hp for what it was, but it was dead reliable.
I'm rambling. Here's my bottom line. Hp limits on caps/ blocks are directly proportional to the WAY you make the power. I get the feeling that aluminum caps give a false sense of security because of the missing evidence of movement on the surface, but I do agree that they are a better/simpler choice then using factory caps. The loss of evidence on the mating surfaces, had I used aluminum... would have lead me to a catastrophic engine failure for sure. I never would have learned the things I have about the tune up.
Take a look at the bottom end. Factory 400 block, factory caps with tool steel straps, factory crank. 1100 hp. Other than freshening every 2 years, rings bearings and wrist pins, the bottom end was together from 1992 until the time I sold the car in 2006. I'm NOT advocating the goofy straps I used. Just showing what you can get away with if you veer off from the "rules" of engine building/tuning.

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