We hypothesize that Thela was like the size of Mars. Since all orbits in the solar system are due to gravity (assuming no rocket powered alien space rocks), we can calculate the maximum velocity a planetesimal would have as if it were falling from infinity into the sun. I can't be asked to do this right now, but based on the Virial Theorem and Earth's orbital velocity of 30 km/s, I bet ~60 km/s is the theoretical maximum impact speed. Mars has 10% of Earth's mass, so at best Thela would have 20% of the Earth's momentum.
And more likely the impact occurred in a grazing fashion in order to make the moon and with much less velocity, since it was probably already in an orbit around the sun. So back of the envelope, we wouldn't expect much to happen to Earth orbit other than a small disturbance that makes it more elliptical for a time. Then over the next few Gyr the solar tidal interactions circularize it.