Swapped my OEM front wishbone bushes for the poly ones yesterday. After 28k (8k with the WALK kit) and 2 trackdays/4 days at The 'Ring they were in A1 condition.
I don't think anyone with the OEM bushes and a WALK currently fitted has anything to worry about, I think you'll get average wear rates. The only reason to upgrade is the feel, as it's a little tighter, as you'd expect, and turn in is improved further still.
If you're still to do the mod however, go fully poly. You'll need the WALK and Powerflex front wishbone bushes, part number PFF85-501 £21.91inc VAT.
If anyone tries telling you that you need to buy the whole wishbone kit (i.e. 4 bushes) then they either don't know any better or they're telling you porkies... you don't.