not too much as my guess is you'll just endup 'confusing' the ECU.
Then, my 2nd guess will be that the ECU will compensate to make it what it should be.
Finally, I think that there is more to do on a Electronic controled engine than just move forward the timing.
Isn't that why kids get them 'chips' put in (to control timing AND fuel/air intake etc..)?
You ain't dealing with a simple early 80's GM engine anymore!