i used T8 on 2/11/15 at around 3:45 PM--the lines for pre-check were as bad then as in the moning; they had only ONE agent checking id and docs (so that line had gown to over 20 people)--they made an agent change+ opened another check podium...that helped.
In LAS pre-check is much better, but not like it was when p-c first started, where I would be the ONLY one on line.
In theory, P-C is a great system, but the way they do at JFK is a disaster.