It can get better because the way the load and conditions change on the street at various throttle amounts has never been best served wiht fixed ignition.
If you had a coil near plug setup like an LS motor and an electronic ignition map that you could tune- like with something similar to this:
http://www.msdignition.com/Products/Ignitions/Distributorless_Ignitions/Ignitions/6010_-_MSD_6LS_Ignition_Controller_for_LS1-LS6_(24_tooth_crank_trigger)_Engines.aspx
You could then dyno tune the car and really nail down a decent ignition tune.
I'm sorry i can't be more specific because I don't know where you were with the MSD distributor tunes. The certainly can be some challenges on what you are doing. Using box stock MSD distributors aren't really a great start. You probably had to change the MA busing to allow you to have more Intitial advance without being over timed with your total. Sometimes you want the total in by 3k but lightweight springs cause inconsistant timing so you have to stiffen up the springs and make the weights heavier.
Then there is the issue of the VA. What one did you start with? There are adjustable ones, but typically the timing amount isn't adjustable- only the Vacuum diaphram.
One thing to note is that GM supplies their 572 eninges with distributors that include MA. Admitedly, the 720 hp only has MA, but the 620hp even has a VA.
