Throwing off the exchange generated timestamps doesn't mess with any HFTs. The exchanges themselves don't synchronize against one another so the timestamps are basically useless to begin with.
I think it is outlandish to suggest, but in theory you could time a GPS attack long enough and timestamping requirements tight enough to make the drift a problem.
I can't think of a single real use case for that level of timestamping accuracy nor a reason you would want to mess with it. But you could.
But it isn't just about the accuracy it is about the immediate degrees of drift that can occur. With GPS time disciplining removed a crap OCXO (and there are plenty) will wander off back and forth in mad errant degrees. With this used to discipline dozens of other hosts results are not always pleasant.
