Thanks that's good info the EXT appears to be working - I didn't realise that existed. And knowing that the ARB one will likely will be supported on our minimum spec Radeon 5870 means I can take the risk to wrap that as you say to remove later when the drivers expose the ARB version.
I imagine if the Radeon 5870 supported OpenGL 4.5 fully without performance issues, and then they made a 4.6 that it supported, that depreciated all the non DSA to error that would be a great boon to developers confusion!