I am trying to use the AMD R9 280X (Tahiti) with the latest Ubuntu 18.04 and
AMDGPU-PRO driver.

OpenCL appears to work with 

export GPU_FORCE_64BIT_PTR=1

see https://bugs.freedesktop.org/show_bug.cgi?id=102491

however the memory appears corrupted on the device resulting in random data
populating partially or entirely the buffers.

A simple OpenCL program is sufficient to show the problem:

https://github.com/rdemaria/opencl_simpletest

in my case.