There are reasons .. The S&S pump will scavenge more oil , less aeration induced into the oil , better oil pressure at idle , and less peak pressure, more pressure is more power to turn the pump , less oil in cranks case is free power, cam plate has bushings in it better wear, better oil control due to pinion fitment  and plate is more ridge which will aid in reducing  pinion wobble which directly effects cam timing.   If you have the money I feel its a well spent upgrade. If not the stock parts do work and its not like you are going backwards. 
Pressure relief valve on S&S cam plate has been set at the factory 
to relieve at approximately 40 psi. Using a caliper note the distance 
from the end of the pressure relief screw to the pressure relief nut 
and note this dimension for future reference. 
     The  pressure  relief  valve  is  designed  to  keep  the  pump  from  
building excessive oil pressure at high RPM’s. Resist the temptation 
to turn the screw in to try and raise the low rpm oil pressure. If your 
engine has no oil pressure at idle, turning the pressure relief screw 
in more will normally not help this condition, as this pressure will 
be below the operating point of the pressure relief valve. If you do 
adjust  the  pressure  relief  valve  to  something  other  than  the  
factory setting. We recommend not turning it more than 1/2 turn at 
a time. One half turn of the screw will move the adjuster .025"which 
typically  changes  oil  pressure  approximately  4  psi,  but  other  
factors may effect the actual change in pressure.
Turning  the  pressure  adjusting  screw  in  more  than  necessary  will  
cause  high  oil  pressure  at  high  RPM’s  but  will  also  needlessly  raise  oil  
temperature  and  rob  horsepower  without  really  raising  low  rpm  oil  
pressure.  Ideally  you  want  to  keep  your  oil  pressure  as  low  as  you  can  
while keeping the tappets quiet. 
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