Edit: let's try and research if a custom timing advance cover could improve the timing much. If it might be helpful, I might be able to design and get a batch of them CNC machined but initial cost is high (It's at or above $100 and that cost doesn't seem to matter how many are made!) so the cost per unit goes down dramatically if we were to have a number of them made. But I would have to be smart in the deisgn so I don't have 100 non-useful covers made up. :?
Dr Diesel has a small amount (less than 1 mm IIRC) shaved off the back end of his engine's injector pintals. This allows faster injection since when the injector opens less of the tip of the pintal is obstructing the injector hole. The modification is not related to timing, per se.