Xantid Swarm is a beating against Keeper, because I usually side out my Swords and Matrices for Red Blasts and Dwarven Miners, which is the right thing to do. I would MUCH rather see Red Blasts brought in against me than Swarms. Red Blast is not even that good against Keeper because many of the key spells are not even Blue (Decree, Scrying, Red Blasts, Shaman, Miners, etc...). Thus, I can deal with Red Blasts easily, but Swarms give me a huge problem since I board out creature removal.
BUT, Red Blast is better in the mirror and other decks than Swarm. Either way, the Swarm is a problem, but I think Red Blasts will keep the opposing Tog deck from going off better than a Swarm would.
Thus, if you expect Keeper, Swarms are amazing. But if there is no Keeper around, I think Red Blast is a better SB option. Just my opinion.
You shouldn't have a problem beating keeper anyway.
I posed this question to a teammate on the issue of xantid swarm vs. duress yesterday:
'If you duressed me turn 1, would you take my fire/ice?'
Keeper runs 8 counters and it uses them primarily to protect its own spells. Keeper can't prevent its opponent from resolving spells, it lacks the counter base. It is because of this that Hulk is traditionally such a bad matchup for keeper. There are just too many draw spells and counters for keeper to shut down. This is also why Xantid Swarm isn't the best choice against keeper, you don't shut down ENOUGH with the conditional swarm. While swarm is sometimes powerful I have found it inconsistent and hardly game-breaking in an already very positive matchup for Hulk.
Personally I don't even side all that many REBS, and I play in an extremely control heavy meta. Siding 3-4 Deep Analysis is the surest bet because they will be useful against Keeper and game-breaking in the mirror. Xantid Swarm has a very unpredictable and situational value in the mirror (highly dependent on the competing builds of the deck)
Incidentally Keeper can turn around its Hulk matchup by boarding in a bullet to stop all of Hulk's draw dead... but that's another thread.