Ok I've ran the 1257 straight lipo in my 8th scale with no problems. My throttle (Futaba 9452) servo got warm but not "hot". I went back to a nimh just to be safe but I'm going to try it again this coming weekend
with different throttle servo. I will report then.
Funny thing is I run a 1s lipo in 12th scale and now in an electric truck for my son. I use a 2s lipo straight to power RX and steering servo, so the servo gets 7.4v. No problem at all.