After the swap of the WP, stat and temp gauge sender unit my car heats up about as quick as it did before.
I think you're possibly on the right track by suspecting your stat might be failing or leaking.
FWIW, last night I put in the old temp gauge sender unit and same results.
Since I don't have a OBD-II scanner I might try to use my infra-red heat gun and see what kind of reading I get off the stat housing unit after its fully warmed up.
Before the swap, it was showing approx 190F after a 20min drive.
