Yeah what is up with the HPI RTR servo? SF-50WP had weird claims online back when introduced by HPI as early as 2012. The reason I know this is it was the stock RTR servo I my 1/8 Apache C1 Flux I've sold already a year ago.
Try adding an external Castle 10A BEC and remove the center wire off the Esc to bypass the esc's onboard BEC. I am not sure which rebranded Esc HPI is using in the Sprint 3 Flux I have coming in this afternoon or early morning tomorrow but I'll take pics.
Either way other solution is to run the cheapest Savox metal gear digital waterproof servo like the SW-0231.
http://www.ebay.com/itm/NEW-Savox-Hi...25.m3641.l6368
I have that on my Bullet ST Flux no issues at all turning those big 2.8" tires even when not in motion. That servo should make easy work of any 1/10 TC the setup.