Edelbrock sells five levels of performance (top end kits) which are listed as:
Performer Torker II Performer RPM Victor Jr. Super Victor+
0.7 - 0.9 0.8 - 1.1 0.9 - 1.3 1.2 - 1.7 1.6 - 2.0
They offer a return on investment of a multiplier you multiply against your engine's displacement. So a 283 with a Performer series package (including Performer or stock heads, Performer or stock intake, and a small 4bbl carb and their performer cam will yield 198 to 248 horsepower. If you bought the Performer RPM kit (once again Performer RPM intake, Performer RPM heads, and cam) then your 283 is guaranteed to pump out between 248 and 282 horsepower. With the most radical kit the Super Victor your 283 would not be very steetable but it would crank out 453 to 566 horsepower (the variance is based upon compression ratio).
Where people get into trouble is in mixing things they shouldn't such a small heads and a big cam that won't get you anything but a head ache. Like I said the Performer is just your stock Chevy Fuelie heads only cast out of aluminum, and the Performer intake is just the same as the Z/28 intake manifold that Chevy used to sell. The cam is a hydraulic version of a Chevy hot cam you could find in Chevrolet Service Parts Power Catalog, though admittedly the Chevy cam would have been a solid back then with the specs of the newer hydraulics sold today.