An engine will make best use of the cylinder pressure (convert it to HP) if peak cylinder pressure is reached at it's "sweet spot", typically between 15-20*
ATDC, to achieve this, most N/A engines (wedge combustion chamber) will need 36-38*
BTDC as an ignition point.
The trick here is not to add as much timing as you can, but rather to add enough to maintain the "sweet spot"... As you add boost (or nitrous) the burn rate will go up, so less timing lead is needed to maintain the sweet spot(lighting it latter still gives the peak pressure at the same * ATDC), adding too much timing will bring peak cylinder pressure closer and closer to TDC, at which point ALL of your cylinder pressure will be trying to blow the crank out of the bottom of the block, or, if you're really lucky, run the engine backwards!
It's a balancing act like running higher compression on pump gas... You
could run 12:1, but with so little timing as to negate any gains the compression gave you and most likely reduce the power you could have had, if you had a lower compression and more/correct ignition advance...
Same thing for boost, if the higher boost results in unstable combustion (say really pushing a T1 Mitsu w/no intercooler), you would make more HP with less boost and proper timing...
Clear as Mud, right?
Mike