There is almost zero gain from 15G to 16T. I wouldn't waste money on it.
+1! There's what? Barely 50 to 60 CFM difference.
When I had the 15g in, it spooled mucho quick. Should I sacrifice spool time for some HP gains?
Depends on what you want, how you want the car to drive, and what you have.
Something that's been known forever is that where the power is can make a vehicle easier to drive/ride quickly. More bottom and and mid-range means you don't have to worry about shifting as much, assuming you have a stick.
If you have an auto and you're not drag racing, a more responsive engine is always the way to go.
OTOH, you can get away with a stick and a larger turbo as you'll have more control over where you are in the rev range.
I prefer a
faster spooling setup every time because where and how I like to drive. Country roads, mountain roads, that kind of 5h1t. Rather than having to focus some thought on being in the right gear with a stick, I can focus more on reading the road/situation, line choice, corner entry, and transition back to power.
If you already have a 15g, that's easily upgraded to a 18t, 19t, and even a 22t. Next step up from there if you want more power but would still like some spool (and not break the bank) would be a .48 AR Stg III or Stg V hotside T3/T4.
Yet a third option that everyone here hates is the TD05-12b. Easily built up to a 16g, 18g, and even 20g. They'll knock you because the manifold required is inferior (and it's true) but reported difference in gains was only around 10hp? Not much to worry about and easily accounted for with smart cam choice and the fact that Mitsu G wheels CRUSH those T wheels on the TD04's.
Think about what kind of driving you want to do then go from there and have fun.