Hello and welcome.
The difference between the two is that MPFI uses what's called batch fire. Basically, the system fires all the injectors for one bank of the engine at once. . SFI fires the injectors individually, so they're only firing the injector at the cylinder that needs fuel at a given moment.
SFI is more precise for better power and mileage.
Yes, the 5.3 is a very good engine.
There are many aftermarket mods that can be done.
K and N air filters and air intake kits, colds air intake kits,
computer chips, etc. Here is a link to one such system...