Look up the Perrin H6 TT. Essentially, you will have 1.25liters per turbo, meaning your turbos will be super small if you want to avoid lag. The 2.5l is already a small engine as is. On Nasioc, you will see many comparisons of spool from 2.5 to 2.0 liter. That will give you a great indication as to what you could get away with, though you will need to extrapolate.
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