0.15 isn't a speed demon by far. The 5955s that you mentioned are really robot servos. The 5995s replaced the 5955s because the 5955s were blowing their FETs under normal operation. Seems like the 5955s had a breakdown issue which is very well documented on robot forums.

If you're going for steering, I always love faster servos. For robotics, a 0.1 to a 0.15 second difference isn't really noticeable but in RC steering, it actually does.

I'd go for a Team Associated DS1313 servo. Best bang for your buck and it's fast as hell. More then enough for even 1/8th scalers.