For a target power of 400-440 bhp at the flywheel, with (probably) a GT(X?)2871R turbo, and 3.8bar FPR and Walbro pump already fitted, what injector size should I choose?

I was thinking that the Bosch green 440cc injectors would be OK, but the 20vt Tuning Workshop on here says the following:

So with the 440's I might only just get to 400bhp, and Siemens appear to be a safer bet to comfortably exceed 400, but are perhaps overkill?

According to this injector calculator with a 5 cylinder turbo, 55psi fuel pressure, 550cc injectors at 90% duty would give 440bhp.

The same calculator says that 440cc would give 393bhp when maxxed out at 100% duty.

Above 440cc is it only worth considering Siemens 630? What about Bosch or Genesis 550?

And before anyone says "plenty of info on the web" well yes, there's too much, and it's too confusing, I want to benefit from the forum knowledge!

As Flea will be doing the mapping what's his advice on this?

He's not said, I'll have to ask again, but I'm interested also in other opinions!

Go for something larger, in case you have a bit of scope creep

So 630cc then? Do I need "spacers" or any other extra bits to be able to fit them direct in place of the originals?

They're direct replacement for OE so nothing else needed, except maybe confirmation from Flea before you purchase.