A single-threaded application will run at full speed in the core it's assigned to. That has always been true -- even in the original Pentium single-core CPU's with hyper-threading.
What you're probably thinking of is that if you had multiple applications running, the single core would be shared between them (since the CPU treated the hyper-threaded CPU as if it had two cores) ... so the individual applications were sharing the core. That, of course, is still true ==> if you have an application mix that has all eight "cores" in use (even though there are only four actual cores), then the processing load will be distributed among those cores.
Bottom line: If there aren't competing demands for the core, a single-threaded app will run at full speed. You can force this to be the case by using processor affinity settings in Task Manager so no other application is assigned to the same core.
