It has a high degree of zoning, and seems a bit dark in color. That being said, Tan often has good pricing, although I've seen some things on his sites which I feel are overpriced too. Overall, he has some good deals, and I have yet to hear of a sapphire being returned because it was found to be either synthetic or diffused.
I have two African sapphires from Tan, and I have to say, I was pleasantly surprised by them, and they turned out to be great buys, and I obtained AGL reports which tested for diffusion, and they both were just heated (not diffused).
I didn't even take cutting into account, because most sapphires are poorly cut since they often don't let the rough out of various sapphire producing countries for cutting. I see plenty of poorly cut sapphires that are expensive, but have great color nonetheless. From what I've personally seen, Tan often doesn't discount for poor cutting, but he discounts for inclusions and/or poor color. He mostly discounts for inclusions though IMHO.
Here's an example of what I mean. Poorly cut sapphire with mediocre color, but it's clean, untreated and large, so it's pretty expensive.