Steve,
Nice to hear from you again.
I've hunted some pretty warm bedrock with the Sand Shark, and I found some nice nuggets with it in those conditions--none were sub-gram though. As well, you must have different hot rocks than the area I worked with the Shark, as hot rocks just weren't a factor. However, a friend of mine has a claim that's loaded with hot rocks that are plenty of trouble! Different areas, different challenges.
The 5000 is great at ignoring most troublesome rocks, and it runs very, very quiet. I can't believe the difference vs. my old 2100 platform--light years of difference, but the 2100 flushed up a lot of nuggets in ground that ate VLF's alive.
I've never used an Infinium, but it has more flexibility than the Sand Shark, that's for sure, and you can change coils, something you can't do with the Shark.
This whole thread has turned out to be very informative.
All the best,
Lanny