now there are 3 hotside sizes available and as Fp explained back pressure is increased very very little,
what would the max whp drop by with each reduction in hotside size?
i.e
the 10.5 will give ***whp
the 9.8 will give ***whp
the 9.0 will give ***whp

 

edited:
the 10.5 will give the most whp
the 9.8 will give between the 10.5 and 9.0 whp
the 9.0 will give the least whp

In reality, it really depends on your setup.