I recently had the opportunity to strip and upgrade a TM Sig 552 belonging to a fellow airsofter so i decided to take some pictures while i was at it and post them here with his kind permission.
Firstly, to start off here is the Sig 552 itself with the top part of the foregrip removed, and also the battery tray has been removed:
Remove the motor plate from the bottom of the grip by unscrewing the two screws either side, you can leave the allen key in the middle alone, this is the motor adjustment plate and unless your motor isn’t meshing correctly with the bevel gear, doesnt need to be touched.
Once that’s done, unhook the wires from the motor itself and remove the motor, watch out for the small metal disc on the bottom of the motor which will no doubt fall off and can sometimes stick to the side of the motor due to magnetism 
next, have a look into the grip and you can see two screws which hold the grip onto the gearbox (brass here, but some aegs they are painted black and very hard to see)
Note the two motor wires go through the same hole in the grip
Fold the stock and remove the screw at the top pictured here
Then remove the two screws on either side of the body just behind the selector switch
the entire stock can then be slid straight off
On the underside of the selector switches is a small allen head grub screw, unscrew these halfway (so you dont lose them) and remove the selectors
underneath the selector is another piece which attaches to the selector gear assembly on the gearbox, unscrew these and lift them off, note that the one on the left hand side has a small spring and plastic piece which click into place when you turn the selector, be careful not to lose these. Also the one on the right hand side has a longer screw
Now remove the screw inside the mag well
And the two screws on either side (at this point you should have removed the bottom half of the foregrip too)
Next remove the screw just forward of the mag well
at this point you should be able to lift the upper half of the body forward and up away from the bottom half, note that it will be necessary also to remove the cable ties holding the fuse holder in place
The gearbox can now be lifted out of the rest of the body
note the position of the gears outside the gearbox which are controlled by the selector switch
right side
and left side
The larger of the gears will simply lift straight out, the small one on either side requires a screw to be removed on the left side and then slides out
Once you remove the gears on both sides, the black plastic piece underneath needs to be remove on the right hand side, simply pull it slightly down towards the trigger and it will unclip, underneath this there is also a small metal piece which sits partially inside the gearbox to block the trigger, unfortunately i havent got a photo of it, but you’ll know it when you see it
This is the positioning of the selector plate and other components, you dont need to remove anything here, this is just for reference
Inside the gearbox with all the original parts installed
Systema High Speed gears, 1 Joule spring and metal bushings installed, also gears were re-shimmed
Now just stick everything back together in reverse order 
The 9.6v custom battery being test fitted without the top half of the foregrip, note the position of the connector and the orientation of the battery with particular reference to the side which only has 2 cells
When the battery is correctly position as above, the only modification that needs to be made to the inside of the foregrip is to dremel away a little bit of the protruding area of one of the nuts/screws that hold the side rail on
and your done, this particular AEG now runs at a nice 19-20 rounds per second and has a nice crisp trigger response, havent had the opportunity to chrono it yet but with a systema 1J spring i would expect in the region of 300-300