i just rang halfords mate, it doesnt list different shocks for the 600 and 800, so they must be the same...but it does list a different coil spring for the the 800 to the 600.
in my opinion you could upgrade the springs and be done...but you need to check that i DO NOT KNOW
anyway, the springs would cost around £100, and if you fit them (health and saftey alert) be careful! me and my mate had a coil come off when we were unsrewing it, we both jumped on it lol...its couldve been worse. they go off like a loaded gun...im not being gay telling you this, its just i know what can happen