There is an easier way to do this and do it correctly. If you use a spoiler you will affect all the wrong aero characteristics. Add fuel tanks with no useable fuel. For example, to model a 1000lb payload, position the tank using the X,Y,Z coordinates in the air file and set total and unusable quantities to 1000.
Then using a fuel gauge or simconnect, set it to read the presence of the item and set the tank full if present or zero if not. This will load the wing center of lift, the trim and control surfaces, and all the other things the are needed to create accurate simulated flight dynamics from the model....assuming the air file is accurate in the first place.
All that being said, I have never tested the payload system to see if it models weight correctly, but I will say this...
IF the sim is adding the correct weight to the aircraft when weapons are added, and in the correct position, AND you're performance is different from the real airplane, then YOUR flight dynamics are wrong. The sim is a math engine using real world math. It only has a few specific bugs that the original ACES team got wrong. All the fundamentals are 100% correct and when the air file is correct the airplane will output identical data to the real airplane within 1 or 2%. Using trickery like invisible spoilers should never be needed unless the function is outside the scope of the sim....otherwise you are getting further from accuracy than you think.