Yes I have installed a TPI setup in my C10 years ago, an important point of early GM EFI is setting the distributor at 0 degrees because the ECM does ALL of the spark advance including at startup. If you set the distributor at 4 degrees static for example, with no programming changes then the ECM is going to assume that is 0 degrees and advance from there. So you'll be off by 4 over the whole map. The ECM has no clue what you've done, it's a computer following a pre-programmed set of instructions, unless you go in there and change those instructions.
Also as far as I know there is no timing tabs on a stock LT1. So no way to know what advance you've set. On the TBI engines there was a connector to disconnect so the ECM would revert to base timing and make no changes while you were setting the timing. I don't think LT1 have that connector. Sure you could hack it and all, the seat of your pants feel good, etc. But engine damage is very possible, crappy fuel mileage, etc.
Oh and just because an aftermarket company put out a part for your car, doesn't mean it's good. OEMs sink millions upon millions into R&D to make sure they put a good reliable product out there. Aftermarket companies pump out parts as fast as possible to make money, they aren't on the hook for lemon laws, NHTSA safety recalls, and so on. So shop with care.