I believe it's simply the HPI pumps more amps than the Tamiya. You may lose a bit of run time, but more speed is almost always worth it.
My brother purchased a Futaba 203 and than upgraded again to a (Futaba) 802 ESC I got the 203, boosted my speed but not as much as the 802. He always stayed that bit quicker than me in otherwise identical cars. The only difference was the colour of paint on the bodies. Both TT01s on max gearing with a silver can.
Quite frustrating as I didn't have the money to keep up.
RCs are like potato chips you can't stop at just one.