Being a former 2560 owner, it is all you need for a stock motor, and it is a great turbo. It is fully capable of blowing up your ****. Ask me how I know. And the sweet spot is indeed 15 psi. It really came alive raising it from 12 to 15; it even sounds better.
My experience with turbo choice is to choose a turbo that can achieve your hp/torque objectives with about 30hp headroom.
On my 1.6L (with headwork) the 2560 had a 'snappy' response and happily made 260bhp at 16psi. A nice turbo. I moved to a 2860rs (0.86A/R turbine) and it felt sluggish when getting on it, but made 280bhp+ at the same boost level.
Later, the same GT2860rs felt great on my 2L build, 'snappy' again and capable of 320 to 340bhp. Near perfect. For some reason I fancied more power and opted for a custom GT3076r (a blend of the 71 and 76 with a larger ported wastegate) so that I could push for 450bhp, it feels like the 2860RS on the 1.6L, a little sluggish, but makes plenty of power. I'm going to dump it soon and spring for something a little smaller, maybe even go back to the 2860rs. On my daily driver 300 to 350bhp is plenty.
BTW on all these turbos spool was only 200rpm slower (boost vs rpm) on the larger units (at most), so they actually made near the same torque at a given rpm, the larger turbos just didn't feel as snappy when getting on it.
Edit: FWIW the GT3076r spools in 5th gear as follows:
120kPa by 2150rpm
140kPa by 2600rpm
160kPa by 3050rpm
180kPa by 3350rpm
This thread is a good illustration of the boost obsession. You guys are never satisfied. I understand the love for a specific vehicle ie. the Miata but thousands of dollars for built motors, going through 2 even 3 turbos on the same car. A car worth less than the turbo setup. This is madness! Do what I did and buy an Evo - 19.5psi factory.
Then again, I have yet to pilot a turbo Miata so what do I know?
no, actually I know exactly what I want and I almost got there but had to rethink a couple parts. I went in circles on my turbo vw, but on this car I knew exactly what I wanted from the start and didn't buy too much stuff twice.