OM-5 Extended ISO Question...

I typically have my OM-5 set at ISO 200 but I recently decided to allow it to adjust the ISO automatically. I set the upper limit at 800 (not very high I know). I noticed that outdoors on a sunny day, the ISO dropped to 64. I was using Program mode so it chose the largest aperture for the lens which was f/4. The shutter speed was 1/8000 and so it appears that to get the correct exposure it had to drop the ISO.

My question is this, is there any detrement to using, or allowing the camera to set, that low extended ISO? I've not had a chance to shoot again with those same conditions so I'm not able to analyze the quality of photos it might produce. So until I can, I thought I'd ask the experts here.

Thanks.

HAND.

Meaning it's easier to have blown out highlights correct? Could this be compensated for in a high dynamic range scene by spot metering on the highlighted area and then boosting shadows in post? It would seem that if I've got less noise by virtue of the <200 ISO that I'd have more room to boost highlights.