How far and in what ways do you see usurpation as a central concern of "The Tempest"?
It undoubtedly is a central concern of the play. Prospero, of course, as Duke of Milan, was usurped by his brother Antonio long before the play began - which is how he got to the island in the first place. Yet when he got there, he usurped the island's natural inhabitants. Here's one of the play's most famous speeches from Caliban, arguing that Prospero has stolen his island:
This island's mine, by Sycorax my mother,
Which thou takest from me. When thou camest first,
Thou strokedst me and madest much of me, wouldst give me
Water with berries in't, and teach me how
To name the bigger light, and how the less,
That burn by day and night: and then I loved thee
And show'd thee all the qualities o' the isle,
The fresh springs, brine-pits, barren place and fertile:
Cursed be I that did so! All the charms
Of Sycorax, toads, beetles, bats, light on you!
For I am all the subjects that you have,
Which first was mine own king: and here you sty me
In this hard rock, whiles you do keep from me
The rest o' the island.
Prospero is usurped, but usurps Caliban. Caliban longs to usurp him back, putting Trinculo and Stephano in charge (they too then become usurpers). Antonio and Sebastian consider killing the king and taking his power.
Thematically, there are even mini usurpations. Caliban wants to father children on Miranda. Even Gonzalo thinks about what would happen if he were king fo the island:
Had I plantation of this isle, my lord,--
He'ld sow't with nettle-seed.
Or docks, or mallows.
And were the king on't, what would I do?
'Scape being drunk for want of wine.
I' the commonwealth I would by contraries
Execute all things; for no kind of traffic
Would I admit...
So, in short, it's absolutely central. You might even describe it as the pattern of the whole play.