I've seen the > 100% utilization for a CPU and only was able to observe that when HT was enabled on the host.  Since HT does show as a core, but only provides, at most, 50% increase in performance per core, I always have assumed it was due to this and the lack of ability to determine if the core being run on is a logical or physical thread, which would have to influence the calculation.

Showing that to users I've always been able to say "See, you don't need as many cores in a VM because I can drive a CPU past 100%!".  It's easier than explaining co-stop, single-threaded apps, etc.

Also, if you are seeing a single core on a 4 core being pegged, you definitely are running a large single-threaded application, and I will bet you see high co-stop values as well.  I would reduce to 2 cores and monitor, and if the aggregate of both cores is between 75% and 85% over a 30 day period, you are optimal in that configuration.  I say 30 day because you want to account for odd spikes in workload and patching.