Let's get this right out of the way: Junior Siavii is not Colin Cole.
Despite playing in a 4-3, Cole is a two-gap style lineman. He controls rather than penetrates and if he has an asset, it's his ability to shed blocks and tackle even while being driven back. It's not pretty, and, in total, it's probably not even very beneficial, but Cole controls. He is a big man with lots of upper body strength.
Siavii backed up Jay Ratliff, and so though he played in a 3-4, he is a one-gap style lineman. He has good initial quickness and is able to plug through that quickness. When Siavii gets a good jump, he can win leverage and hold ground against a double team by being slippery, quick and hard to block. Despite his height, he gets low and powers up and into blockers.
Siavii does not have the leg drive to bull rush. He has active hands and does a good job of disengaging, negotiating a blocker and, sometimes, cutting through and into the backfield. Cole struggles with being blown back. Siavii will struggle with being moved laterally and away from the ball carrier.
I do not see Siavii as an anchor. He is more of a 3-4 end type. Siavii carries his weight in his limbs and through his wide frame. Cole is barrel chested. Cole is the traditional "big man." Blocking Cole is like moving a blocking sled; blocking Siavii is like wrestling an octopus. Cole is rarely knocked out of the play entirely. Siavii will be. At the same time, Siavii is probably superior to Cole at holding against a single block and getting a hand on a passing rusher.
This is a decent acquisition. I would prefer Seattle to have moved E.J. Wilson to the practice squad, if possible, retained Vickerson and had Siavii rotate with Bryant. I do not think Siavii is better than Vickerson or a better fit at nose, and when he generates pass rush, it will be through his own efforts rather than collapsing the pocket like most nose tackles do, like Vickerson does. It will be hot and cold.
So, I am not happy about losing Vickerson, and do not think Siavii replaces Vickerson, but I do think Siavii is a good player and can contribute.