You'll need to take them out to fit the backplates especially on a sli setup.
You could technically do it, but I wouldn't recommend it.
Depending on the backplate/block, some reuse most of the screws that hold the block in position, I wouldn't want to risk getting the thermal paste being borked if you do it in situ.
Wait it out m8
GPU sag is a horrible thing
Wouldn't the block be solid enough to prevent sag?
EK-FC Titan X Backplate is very easy to install. First install EK-FC Titan X series water block according to its installation manual. If you have it already
installed, please check the picture below to see which mounting screws are to be left in their position and which are to be removed. Please remove
the six (6) M3x4 DIN7985 screws and their washers as well (marked with arrow).
The backplate for EK blocks goes on after the block anyway.
You'll need to remove (IIRC) 4 screws around it and install the thermal pads.
The 4 screws are on the outer edges of the board so shouldn't affect the thermal paste.
See here for installation guide
It even includes: