I think the image training/mental training may light some issue on where Goku stood in comparison to Freeza and Cell (though it's strange Goku imagined himself as SSJ to beat Freeza). Still, he beat up Cell easily as SSJ, which might not tell much besides a very solid improvement. I believe he was somewhat close to Cell as a SSJ in the Boo arc, like in the 80~90% vicinity, so him growing strong enough to beat Cell that easily may imply be the case if he reached, maybe, his Boo arc SSJ2 level as SSJ in the beginning of Super... Based in Toyotaro's manga, that is. But with Kid Boo, we didn't see the battle, Goku was cut off from his image training right before they'd fight each other. We don't know if Goku would fight as SSJ or if he'd quickly transform to SSJ2 (if his SSJ = Boo arc SSJ2, then his SSJ2 would be on par with Kid Boo by the SEG multipliers)...
Hmm, reviewing the manga though, it seems he charged at Kid Boo as a SSJ, I don't think there was an implication he'd quickly turn into higher states there. So it may be possible SSJ Goku was = Kid Boo at the start of Super... Making his SSJ3, accounting for a 8x power up on top of his SSJ (~ Kid Boo), more or less (assuming a solid 2x gap, which is fair to me, between post RoSaT SSJ Gotenks and SSJ3 Goku) half of SSJ3 Gotenks/Super Boo, more or less. So I could see his beginning of Super SSJ3 being about half as strong as Super Boo, maybe.
That as far as the manga goes.
