As for larger tubes I think it would come down to what thermal medium you used. Likely more smaller tubes would be more efficent.
Yup. Kinda what I hinted at. I think it's kind of comparible to gauges of wire. The thicker the gauge, the more resistance. So in heat-pipe terms, if the tubes are bigger (and effectively thicker), then it will take longer to heat them up, as well as take longer to cool them down--which wouldn't work well in a closed loop system like a HSF. So, if anything, an abundance of smaller tubes would seem to work better than a few large tubes; probably why the CM V8 does a pretty good job; but you still have to factor in the manufacturing process and what the "guts" of the system are--which obviously matters since Xigmatek pretty much owns any other air cooling out there (including other HSFs with HDT) other than the TRUE (which is technically a different story since it's all copper and not really comparable to the Xiggy. If the Xiggy was all copper, it would probably do as well as the TRUE). The thing to note here, with all air cooling, is that you can basically only get a cool as the ambient air temp in the room/case. Any cooler and you have to mess with the hassle of H20, or in extreme cases, mess with dry ice or liquid nitrogen, etc, etc.
I kind of want to know how well a mineral oil submersion would do for heat dissipation...I've seen them built before. I would imagine that if you put your entire system in it, you could keep it cool without much hassle for at least an hour (since it would probably take the mineral oil that long to heat up). After that, it'd be cool to see if a radiator could keep the whole batch cool enough for some extreme OC'ing. Then again, if mineral oil acts like water, it would take quite some time for that liquid to absorb the heat of the CPU/RAM/GPU, and might do more thermal insulating than anything.
Sometimes I just wish I had endless amounts of money to test these things
...would be fun!