Yup! Sprint reused Clear PLMN to give TMobile roaming higher priority. If you notice next to LTE on the notification bar there is a small "R" and SCP sees it as Clearwire B4 (basically that's the PCC your connecting too on TMobile)
Also, you see it as Sprint because all roaming is going through Sprint's back end that's why the ping times are affected adversely.
Btw, nice speeds!