I haven't bought any plastics off him yet (I'm waiting for the IT blue versions to arrive in the new year), but I have bought other stuff off Yuni Parts.
From what I've seen so far, his reproductions are as good as the originals. And if there is any discrepency from the original, it'll be detailed in the eBay ad.
I bought a set of repro '76 yamaha fork gators off him (the ones with the brass vents in the bottom), and when I sat them beside my NOS set, I could hardly tell the difference. They even had the brass vents!
I've had some email discussions with Andrew (Yuni Parts/Yuni's Place), and he's just started getting into other Yamaha plastics. He used to just concentrate on TT/XT stuff. He's bought all the moulds he can find, and gets them injection moulded at a factory he's used many times before and is happy with.
They should be very high quality, but it will depend on the quality of the mould he's bought.
All in all, I reckon it's definitely worth a try. As he gets some cash flow from the moulds he's just bought, he intends getting new moulds made for other Yamaha plastics.