If I understand the rules for signature Wargear correctly, you take the talent and obtain an item of up to 20 requisition. Then you can take signature Wargear (master) and upgrade your original item to a variant costing up to 40 Req (by making it higher quality or by adding modifications).

Does that mean it is impossible to ever obtain something like an Omnissiah Axe, which costs 30 Req? It is too expensive to buy with the lowest signature Wargear talent and there is no lower quality version to be bought before it.

Or would I need to find some awkward logic like buying a combi-tool for 15 Req and then upgrading it to an Omnissiah Axe (which can serve as a combi-tool, I believe)?

"Requisition cost of his Signature Wargear increases to 40. (At the time this Talent is purchased, you may choose to apply additional Upgrades to your existing Signature Wargear, or replace the Signature Wargear item with a new item worth up to 40 Requisition .)" Corebook, p.126

With SW (Master) you simply take a new shining Omnissiah Axe.

I had read p. 126, but it just doesn't seem very clear. The prerequisite for Signature Wargear (Master) says "Signature Wargear for the item selected". Although, I suppose all that really means is that you can't get a second item with the Master talent...

So I would have to get the Signature Wargear talent first, grabbing any item whatsoever? Then, I buy Signature Wargear (Master) and replace that item with an Omnissiah Axe (still benefitting from the bonus effects in Table 4-2)?

This is how I interpret the rules, yes.

Or you could wait and buy both signature wargear and signature wargear(master) at the same time and just grab your axe in one swoop.