Doesn't seem to be true, reference voltage is just very slightly higher if at all:
http://ht4u.net/reviews/2010/powercolor_pcs_plus_hd5870/index4.php
Also, I'm not sure Cypress really clocks higher. At reference voltage both HD4890 and HD5870 often fail to exceed 900Mhz significantly - now of course Cypress has a lower reference voltage but it's a different process so not really directly comparable.
I think noone really tried that much upping the voltage on HD4890 to get higher clocks (I don't know what the safe limits are for that neither for HD4890/HD5870), though some overclockers did and also reached clocks in excess of 1Ghz (often the cards didn't have appropriate cooling for the vrm limiting those experiments).