the cap for 1 sec GCD is around 1268 or thereabouts (assuming you have additional raid buffs). With that said if you given it some thought as to exactly WHICH of your spells will be capped at 1 second, you probably realise that the cap will only affect your Mind Blast and Vampiric Touch cast times. These two spells are the only rotational spells than can fall below 1 second cast time.
The reason why there isn't a cap for shadow priests (or at least not with the current obtainable gear) is because the 1268 cap does NOT cap Mind Flay, which should be your main filler spell, and it also does NOT cap your DoT ticking frequency.
Since the GCD cap only affects your Mind Blast and Vampiric Touch execution times, neither of which is considered significant, reaching it should not stop you from stacking haste.