HyperThreading doesn't really double your processing power. Basically, the 50% is Windows lying to you.

If you migrate to BOINC, you can run work on both virtual CPUs, but the actual performance increase will vary between none and very little.

If you want to give it a try, there are straightforward instructions here: http://www.wcgwiki.org/en/boinc/migration.cfm

That is odd.

Have you used the throttle tool so that UD is using 100% of the physical CPU?

Are you running any other CPU intensive programs?

Don't go by result count. Some results are bigger than others. Use points to compare different computers on a fair basis.

The AMD machines have a better floating point register and clock speed does not exactly correspond to Intel. That means that indeed they can do more with less. Three AMD 2.4GHz machines here provide a total of nearly 70 GC workunits per day.