Req 8 items are in the game already, they're just rare and over-expensive so I don't think it would unbalance anything. It would give people another reason to play in hard mode.
The req 9 green market seems to be about dead anyway, so it shouldn't upset the merchants either.
edit:
A number of people have stated that req 8 versions are somehow overpowered, because pvp chars can't make them. I don't see how making req 8 greens would make this problem any worse.
Currently you can get req 8s in all continents in pve, but they're expensive.
Making them available as greens would give those that don't want to pay for a Gold req 8, a cheaper alternative.
The whole idea behind greens and collectors was to make perfect items available at lower cost to players. Pay for skins, not stats, etc.
Moaning that pvp players can't make the full range of items available to pve players is a different problem. (although it does cut down on the 55 monks in HA
