Nissan Silvia s15 turbo = GT2560r
 

I sort of established that the s15 turbo is a GT2560R.
If that is the case, since the s15 turbo is generally a lot cheaper than the GT2560r
http://cgi.ebay.com/ebaymotors/Genui...#ht_1262wt_722

should I be using that instead? Is there something I m missing out here?

There are a few different S15 turbos but the one with the part number containing 91F00 is the dual ball bearing one that's the same thing as a 2560. More commonly what you'll find for sale is the 69F00 journal bearing type. Either will work fine on a Miata, in fact before I decided to throw out the budget and start shitting hundred dollar bills I was looking for a cheap used S14/15 turbo. The Nissan ones are presumably cheaper because an OEM got a discount for buying a shit ton of them, but that one you linked should be identical to a 2560.

91F00 is the one I am looking at. The only difference I know is that it comes with Nissan inlet and outlet flange. It probably cost <$50 to get the flange made at the intercooler pipes.
At that pricepoint, it is a lot cheaper than 2560, which makes me wonder if I should buy the 2560 or the S15 turbo and hence asking here if there is anything different that I am unaware of.
If there are no difference, I guess the decision is made. Thanks.

You can buy adapters for the compressor inlet and outlet. ATP sells them and there are cheaper ones on ebay too.

Yes they are the same as the GT2560R. You just have to make sure you check the tag like mentioned above. I think they are cheaper since you rarely know the history and they came on cars built 15 years ago so you know it's probably not new. If you live in a Navy town like I do, they tend to pop up occasionally listed as simply a T28. Sailors bring them back from Japan for cheap or are upgrading their SR20DET and getting rid of the stock turbo. I bought one from a guy who was selling his to replace it with a "ball bearing" turbo. He didn't realize he already had one! The key is knowing what to look for and checking the tag. You can find out a ton just googling the part number on the tag. Some of the key things I look for in pics if you can't see the tag are the bolt on compressor housing and the heat shield bracket on the front with two encapsulated nuts.

Clay, I am referring to a brand new Nissan turbo. Most respectable Nissan vendor are selling these substantially cheaper.