From 19.X.X till now all amd gpu drivers support 8gb single memory allocation. You can try installing other driver but your chances are little.
oh, tk u
FIXED: You were right for the driver. Install driver 20.5.1 or later and you will be able to run -clKernel 3. It seems that 8GB allocation is introduced in 20.5.1 driver. I have installed 20.4.2 driver on my computer and got the same message as you "clKernel 3 is ignored", and my "Max allocation size" went down to 4244635648.
tk u, love u so muchhhh...