I have to agree with what has been said so far with regards to the 14mm pump head. The 12mm can supply all the fuel you will need. It seems like you have to address the air side of things. No good pumping the fuel in if it can't burn with anything
First thing I would do would be install a charge cooler. This will make a big difference as numerous people on this board will confirm. (well, personally I'd measure the charge temps before first, nothing like quantifiable data to prove your thinking!) If hardpiping is used, or a high quality silicone hose, there will be no extra lag, just more horsepower
After this, i would dive into the engine. Bare minimum would be a top end build and inspection to ensure the gasket seal is 100%. Then, head stripped out and the head ported. Diesel castings of that era are notoriously poor!! This step alone will REDUCE lag and INCREASE power and torque
On petrol engines, people didn't used to believe me, that I could make more power on less boost than they were using. Boost pressure is a figure that shows how hard it is to push that air in. If i can get that air in easier, the air FLOW will rise, but the boost drop. Of course, if the turbo is capable, easy enough to wind the boost up and get even more power lol! Of course, with more air, you can put more fuel, more power, just watch those EGT's like a hawk!
After that, a bottom end build would be your option. Knowing Jap stuff tho, it wouldn't be to bad as they don't really tend to wear to much! Just a matter of a spot of balancing (which I covered in a post in the TDi section) and basically "blue printing" to ensure everything is 100% and working as well as it possibly can.
After that, i would say with my tongue in cheek get a better driver?
Of course I've missed out a lot of points and parts that would need doing, but it seems you needed a bit of a roadmap to develop the engine.
Sorry for the ramblings, but you've basically come to the end of the bolt on stage. Gotta dig deep, but then if motorsport was cheap, I wouldn't be living in poverty!
-Gavin