How fast can a modern ASIC be clocked?
It depends on the complexity of the 'critical path' - the sequence of logic events (gates triggering, signals being propagated down the ASIC trace/metalization - capacitive and inductive integrity losses). 

For very simple circuits, it can easily reach 100s of GHz - on the other hand, once you start doing something useful with your signals, it becomes more and more bothersome. 

Obviously, such high frequencies can't be delivered on the IO pins - which means you need to start thinking about rather complex PLL systems on-chip (so-called frequency multipliers on CPUs), but, 40Gbit network cards are completely attainable via TSMC, and those tend to communicate with their GBICs at 5-10GHz range.