I does make a difference on the suction side also.
i.e.

If you get your self a coffee stir, and a soda straw, and try to suck
you soda (pop to all you non-Pennsylvanians) through both of them you
will find it harder to suck it through the smaller straw.

So it is with a pump,
the smaller the suction piping the more the pump has to work, thus less
flow.

The smaller the discharge piping the harder the pump has to work. Thus
less flow but more pressure.

--
/\/\ike
Chagoi
http://ourkoipond.com

gng wrote:

Thanks - so the big question is whether it would make any difference at all
if I changed out the input feed tubing with 2" but left the output (to
filter, uv, and waterfall) 1.5". Right now the waterfall is looking great -
should I just forget about it?