Was Raditz faster than the speed of light?
Raditz being FTL was a dub fabrication, it is not present in the original manga.
However i noticed that this question has been discussed on various forums and each of them concluded that one shouldn't expect any realism from a non-science based show (though they said this in a lot less polite manner).

Source- http://www.killermovies.com/forums/archive/index.php/t-527409-lightspeed-in-the-dbzverse.html

Discussions-


https://www.neoseeker.com/forums/88/t2273700-raditz-moving-at-speed-of-light/
https://comicvine.gamespot.com/forums/dragon-ball-universe-1775304/raditz-vs-speed-of-light-1747635/