New Zealand Datsun Club

I have a A15 engine thats got double valve springs, a mild camshaft grind (that supposedly revs to 10k), twin 40mm dellorotos carbs and extractors. And head work done by reg cook (supposedly)

I'm wondering what else I can do to my engine to get more power out of it.

Sounds like it could be a good start point. Are all the mods 'tuned' to work together? How fresh is the engine and has the bottom end been balanced? 10 grand should = HP, but not sure for how long. What flywheel do you have?
Boost will yield power gains, there are a couple in the build on here Wal 2 doors, http://www.nzdatsun.com/forums/viewtopic.php?f=9&t=45" onclick="window.open(this.href);return false;
and James blitzballa. http://www.nzdatsun.com/forums/viewtopic.php?f=9&t=7161" onclick="window.open(this.href);return false;
Carbs sound cool, but HP Costs. I See Gman has just uncovered an up-sized A series. http://www.nzdatsun.com/forums/viewtopic.php?f=2&t=7952" onclick="window.open(this.href);return false;
All the best with your quest.

Forced induction.
Just saying.

You could:
Go mental with the head with huge valves ( titanium) along with springs and retainers.
Roller rockers
Hand made tuned headers ceramic coated
45MM sidedraughts with cold air box
Raise the compression to 13/1 ( avgas only from that point )
High performance cam with adjustable front sprocket to accurately dial it in.
Lightened push rods ( Tomei )
Lightened cam followers ( Tomei again )
Electronic ignition with individual coils.
Forged pistons
After market rods and rod bolts
Knife edge the crank along with opening up the galleries as per the Nissan competition manual specs
Polish the inside of the block
Probably forgotten a heap of other things

OR,
Do as Bart suggested and turbo it.
Have a look on Datsun1200.com,especially on Sikyne 's posts and see just what is easily achievable out of just a stock 1171 motor for not much more than what you could sell your sidedraughts and manifold for and if done well I guarantee you will not be disappointed.
PS "mild" camshaft that revs to 10000rpm- I don't really think so....
Cheers
Dave