Sabretooth isn’t a good character, for one reason. He’s a one trick pony, and that pony isn’t good enough.
If you’re playing Mag vs Sabre, you can bet money on him doing repeated j.fierce -> j.roundhouse, and not a whole lot else. Let him. Each time he hits you with j.fierce, pushblock. He’ll keep doing it, and after 3 or 4 pushblocks, he’ll be too far away to do it. But, guess what, he’ll do it anyway. And miss.(Unless you do something stupid like Dash into it)
So, with that method, you can push Sabre far enough away, where he can’t do his j.fierce lockdown. It opens up a window(Albeit a brief window) , where you can do what you want. And, once Mags can do what he wants in a Mag vs Sabre matchup, Mag controls the rest of the match. Just use sj.roundhouse.
Air vs Air, Mag vs Sabre, Mag wins. He’s more manueverable, and can follow up his hits. Air vs Ground, Mag vs Sabre, Mag wins. Sabre has shitty Anti Air, Mag has mad good AvG. Ground vs Air, Mag vs Sabre, Sabre wins. What this means… Is that, as Mag… Stay in the air, and you win. Spend the whole match SJing up and Air Dashing Down/Down-Towards/Down-Back. SJ up with sj.roundhouse if Sabre is close, and if it hits, cancel into AD D-T and combo into Grav -> Glitch Tempest, or Air Throw into corner into Grav -> Glitch Tempest, ggpo. If it gets block, AD D-T anyway and pressure. If Sabre is far, SJ up and AD D-T with roundhouse or forward. SJ -> sj.roundhouse totally shuts down his jump game. None of his moves outprioritize it unless you’re wicked close, his more powerful air moves are all too slow to beat it out, and if he blocks it, he has to sit in blockstun, fall back, and land, while you AD into his face and pressure, only to do it again, and if it hits, it’s a free tempest at worst, and ggpo at best. Sabre just has such sucky Anti Air. Even if he DOES land a lucky c.fierce, it won’t amount to anything(Except, if he’s smart, you might have to deal with his jump lockdown for a little bit afterwards, but, as explained, just pushblock that shit away), because Mag is a pain in the ass to air throw, due to his stupid air reel. Your air throws are more dangerous, too, just a little.
I think, once you guys get a feel for how to use Mag’s SJ/Air Dash speed to apply pressure, you’ll be able to get around Sabre’s really slow j.fierce all by yourselves. 
Personally, I use Ryu to pwn Sabre-scrubs, because Ryu j.fierce > Sabre’s everything(Plus Jab DP. Oop, Sabre = Lose.)
Also, about that Kaillera stuff… Yeah, I used to play a lot on that… For a few years, I was the XvsSF best there, but I stopped playing when my PC and the servers started getting a little too crappy to play on. As for who’s top now, I couldn’t really tell you, except that T-Kimura and Joker are probably up there.