Special Beam Cannon is the speed of light. It's a beam made out of super charged light. It moves at the speed of light. This cannot be disputed. There's a giant light and all.
Thus Raditz is FTL+ because he reacted to it. He's an FTL timer, easily.
Since even Gohan in the Namek Saga is many times stronger and faster than Raditz, and gets completely outclassed by someone like Zarbon, it's easy to assume how ludicrous speed gets in DBZ.
Now you ask me, why have DBZ characters never consistently shown speed of that level? It's simple, because they were never put in an immediate life or death situation. Raditz flight or fight kicked in and he became FTL.
Since OBD matches are always bloodlusted, they are always FTL.
SSJ Vegeto, for example, would completely shit on Herald Thor who is only x100 times FTL. Vegeto is probably millions of times FTL and I wouldn't be surprised is SSG Goku reaches The Shrike level.
Please start using this in battles, thank you.