My thoughts are on the same wave length... I too think the 1968 Z28 302 fuel pump would be the same or close to the same rate as the 396 Hi-Perf rate (even with an 8.5 psi pressure) since they both run the same Holley carb (#3923289-DZ - [#4053] 800 cfm). I too think the 15 GPH is way to low for either of these engines. (the 15gph flow rate was based off the listed 1 pint in 30 sec. to 45 sec.)
I am thinking just like you, those High Performance Chevy engines stay hungry and you gotta feed em good.
I also think that the SS 396 did use a different fuel pump from that of the Z28 302... but that may be just the difference of SB to BB.gpm = gallons per minute
gph = gallons per hour
The required fuel flow rate might likely be more of a factor and determination based on the engines (heads, intake, cubic inches and exhaust flows & capacities) fuel air consumption flow rates.
You probably already know this stuff but for the others reading these posts a little education (ed-u-ma-kating as some call it)
... might help understand this subject.
I have found this information below in many automotive books...Formulas:
air capacity = (rpm x CID)/2 (divide by 2 for four strokes, ie. 1/2 the cid)
cfm theoretical = ((RPM x CID)/3456)
VE = (cfm actual / cfm theoretical ) x 100
bsfe = fuel (#/hr) / brake horsepower (BHP) therfore => fuel #/hr = bsfe x BHP
Knowing this then.... Fuel #/hr x 6.25 #/gal = gph the flow rate of the pump
# = pounds
Gasoline weight ~= 6.25# at 72 F
1728 = number of cubic inches in a cubic foot (12x12x12)
3456 = 1728 x 2
cfm = cubic feet per minute
CID = cubic inch displacement, for example 302 cubic inches
RPM = maximum RPM,
VE = volumetric efficiency. VE is the relationship between theoretical air capacity and the actual air flow of the engine.
BSFC = (Brake Specific Fuel Consumption), i.e. the maximum fuel system pressure, and the pump's flow volume at that pressure.
As Chick (an expert 68 Z28 member) points out the F.A.S.T. drag racers should have some info on this.
I would also give Jerry MacNeish an email shout out. Jerry is a long time Drag racer (and I think he's quite good at it too) who runs a 67 Z28. Here is his link. http://www.z28camaro.com/
Also another good contact would be Ken Lucas, who has been restoring Camaro's at ConCours levels for years. Here is his link http://www.lucas-restorations.com/
I am sure there are more expert members here at CRG who can give you a better answer than I have. JOE